Regulation of meiotic prophase arrest in mouse oocytes by GPR3, a constitutive activator of the Gs G protein

Exploring orphan GPCRs in neurodegenerative diseases

Neurodegenerative disorders represent a significant and growing health burden worldwide. Unfortunately, limited therapeutic options are currently available despite ongoing efforts. Over the past decades, research efforts have increasingly focused on understanding the molecular mechanisms underlying these devastating conditions. Orphan receptors, a class of receptors with no known endogenous ligands, emerge as promising druggable targets for diverse diseases. This review aims to direct attention to a subgroup of orphan GPCRs, in particular class A orphans that have roles in neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease, and Multiple sclerosis. We highlight the diverse roles orphan receptors play in regulating critical cellular processes such as synaptic transmission, neuronal survival and neuro-inflammation. Moreover, we discuss the therapeutic potential of targeting orphan receptors for the treatment of neurodegenerative disorders, emphasizing recent advances in drug discovery and preclinical studies. Finally, we outline future directions and challenges in orphan receptor research.

Rare variants in GPR3 in POI patients: a case series with review of literature

BACKGROUND

Premature ovarian insufficiency (POI) is a highly heterogeneous disease, and up to 25% of the cases can be explained by genetic causes. G protein-coupled receptor 3 (GPR3) plays an important role in oocyte arrest, and Gpr3-deficient mice exhibited POI-like phenotypes.

CASE PRESENTATION

We identified two heterozygous missense variants of GPR3: NM_005281: c.C973T (p.R325C) and c.G772A (p.A258T) in two sporadic Han Chinese POI cases through whole exome sequencing and genetic analysis. The two patients were diagnosed as POI in their late 20s, presenting elevated serum levels of follicle stimulating hormone and secondary amenorrhea. Both variants are very rare in the population databases of ExAC, gnomAD and PGG.Han. The affected amino acids are conserved across species and the mutated amino acids are predicted deleterious with bioinformatics prediction tools and the protein three-dimensional structure analysis.

CONCLUSIONS

It is the first report of rare GPR3 variants associated with POI women, providing an important piece of evidence for GPR3 as a candidate gene which should be screened in POI. This finding suggested the necessity of including GPR3 in etiology study and genetic counseling of POI patients.

Unraveling the Puzzle: Oocyte Maturation Abnormalities (OMAS)

Oocyte maturation abnormalities (OMAS) are a poorly understood area of reproductive medicine. Much remains to be understood about how OMAS occur. However, current knowledge has provided some insight into the mechanistic and genetic origins of this syndrome. In this study, current classifications of OMAS syndromes are discussed and areas of inadequacy are highlighted. We explain why empty follicle syndrome, dysmorphic oocytes, some types of premature ovarian insufficiency and resistant ovary syndrome can cause OMAS. We discuss live births in different types of OMAS and when subjects can be offered treatment with autologous oocytes. As such, we present this review of the mechanism and understanding of OMAS to better lead the clinician in understanding this difficult-to-treat diagnosis.

G protein-biased GPR3 signaling ameliorates amyloid pathology in a preclinical Alzheimer's disease mouse model

Biased G protein-coupled receptor (GPCR) ligands, which preferentially activate G protein or β-arrestin signaling pathways, are leading to the development of drugs with superior efficacy and reduced side effects in heart disease, pain management, and neuropsychiatric disorders. Although GPCRs are implicated in the pathophysiology of Alzheimer's disease (AD), biased GPCR signaling is a largely unexplored area of investigation in AD. Our previous work demonstrated that GPR3-mediated β-arrestin signaling modulates amyloid-β (Aβ) generation in vitro and that Gpr3 deficiency ameliorates Aβ pathology in vivo. However, Gpr3-deficient mice display several adverse phenotypes, including elevated anxiety-like behavior, reduced fertility, and memory impairment, which are potentially associated with impaired G protein signaling. Here, we generated a G protein-biased GPR3 mouse model to investigate the physiological and pathophysiological consequences of selective elimination of GPR3-mediated β-arrestin signaling in vivo. In contrast to Gpr3-deficient mice, G protein-biased GPR3 mice do not display elevated anxiety levels, reduced fertility, or cognitive impairment. We further determined that G protein-biased signaling reduces soluble Aβ levels and leads to a decrease in the area and compaction of amyloid plaques in the preclinical AppNL-G-F AD mouse model. The changes in amyloid pathology are accompanied by robust microglial and astrocytic hypertrophy, which suggest a protective glial response that may limit amyloid plaque development in G protein-biased GPR3 AD mice. Collectively, these studies indicate that GPR3-mediated G protein and β-arrestin signaling produce discrete and separable effects and provide proof of concept for the development of safer GPCR-targeting therapeutics with more directed pharmacological action for AD.

Neurotransmitters, neuropeptides and calcium in oocyte maturation and early development

A primary reason behind the high level of complexity we embody as multicellular organisms is a highly complex intracellular and intercellular communication system. As a result, the activities of multiple cell types and tissues can be modulated resulting in a specific physiological function. One of the key players in this communication process is extracellular signaling molecules that can act in autocrine, paracrine, and endocrine fashion to regulate distinct physiological responses. Neurotransmitters and neuropeptides are signaling molecules that renders long-range communication possible. In normal conditions, neurotransmitters are involved in normal responses such as development and normal physiological aspects; however, the dysregulation of neurotransmitters mediated signaling has been associated with several pathologies such as neurodegenerative, neurological, psychiatric disorders, and other pathologies. One of the interesting topics that is not yet fully explored is the connection between neuronal signaling and physiological changes during oocyte maturation and fertilization. Knowing the importance of Ca²⁺ signaling in these reproductive processes, our objective in this review is to highlight the link between the neuronal signals and the intracellular changes in calcium during oocyte maturation and embryogenesis. Calcium (Ca²⁺) is a ubiquitous intracellular mediator involved in various cellular functions such as releasing neurotransmitters from neurons, contraction of muscle cells, fertilization, and cell differentiation and morphogenesis. The multiple roles played by this ion in mediating signals can be primarily explained by its spatiotemporal dynamics that are kept tightly checked by mechanisms that control its entry through plasma membrane and its storage on intracellular stores. Given the large electrochemical gradient of the ion across the plasma membrane and intracellular stores, signals that can modulate Ca²⁺ entry channels or Ca²⁺ receptors in the stores will cause Ca²⁺ to be elevated in the cytosol and consequently activating downstream Ca²⁺-responsive proteins resulting in specific cellular responses. This review aims to provide an overview of the reported neurotransmitters and neuropeptides that participate in early stages of development and their association with Ca²⁺ signaling.

Oocyte maturation abnormalities - A systematic review of the evidence and mechanisms in a rare but difficult to manage fertility pheneomina

A small proportion of infertile women experience repeated oocyte maturation abnormalities (OMAS). OMAS include degenerated and dysmorphic oocytes, empty follicle syndrome, oocyte maturation arrest (OMA), resistant ovary syndrome and maturation defects due to primary ovarian insufficiency. Genetic factors play an important role in OMAS but still need specifications. This review documents the spectrum of OMAS and to evaluate the multiple subtypes classified as OMAS. In this review, readers will be able to understand the oocyte maturation mechanism, gene expression and their regulation that lead to different subtypes of OMAs, and it will discuss the animal and human studies related to OMAS and lastly the treatment options for OMAs. Literature searches using PubMed, MEDLINE, Embase, National Institute for Health and Care Excellence were performed to identify articles written in English focusing on Oocyte Maturation Abnormalities by looking for the following relevant keywords. A search was made with the specified keywords and included books and documents, clinical trials, animal studies, human studies, meta-analysis, randomized controlled trials, reviews, systematic reviews and options written in english. The search detected 3,953 sources published from 1961 to 2021. After title and abstract screening for study type, duplicates and relevancy, 2,914 studies were excluded. The remaining 1,039 records were assessed for eligibility by full-text reading and 886 records were then excluded. Two hundred and twenty seven full-text articles and 0 book chapters from the database were selected for inclusion. Overall, 227 articles, one unpublished and one abstract paper were included in this final review. In this review study, OMAS were classified and extensively evaluatedand possible treatment options under the light of current information, present literature and ongoing studies. Either genetic studies or in vitro maturation studies that will be handled in the future will lead more informations to be reached and may make it possible to obtain pregnancies.

Silencing the G-protein coupled receptor 3-salt inducible kinase 2 pathway promotes human β cell proliferation

Loss of pancreatic β cells is the hallmark of type 1 diabetes, for which provision of insulin is the standard of care. While regenerative and stem cell therapies hold the promise of generating single-source or host-matched tissue to obviate immune-mediated complications, these will still require surgical intervention and immunosuppression. Here we report the development of a high-throughput RNAi screening approach to identify upstream pathways that regulate adult human β cell quiescence and demonstrate in a screen of the GPCRome that silencing G-protein coupled receptor 3 (GPR3) leads to human pancreatic β cell proliferation. Loss of GPR3 leads to activation of Salt Inducible Kinase 2 (SIK2), which is necessary and sufficient to drive cell cycle entry, increase β cell mass, and enhance insulin secretion in mice. Taken together, our data show that targeting the GPR3-SIK2 pathway is a potential strategy to stimulate the regeneration of β cells.

Mechanisms of Oocyte Maturation and Related Epigenetic Regulation

Meiosis is the basis of sexual reproduction. In female mammals, meiosis of oocytes starts before birth and sustains at the dictyate stage of meiotic prophase I before gonadotropins-induced ovulation happens. Once meiosis gets started, the oocytes undergo the leptotene, zygotene, and pachytene stages, and then arrest at the dictyate stage. During each estrus cycle in mammals, or menstrual cycle in humans, a small portion of oocytes within preovulatory follicles may resume meiosis. It is crucial for females to supply high quality mature oocytes for sustaining fertility, which is generally achieved by fine-tuning oocyte meiotic arrest and resumption progression. Anything that disturbs the process may result in failure of oogenesis and seriously affect both the fertility and the health of females. Therefore, uncovering the regulatory network of oocyte meiosis progression illuminates not only how the foundations of mammalian reproduction are laid, but how mis-regulation of these steps result in infertility. In order to provide an overview of the recently uncovered cellular and molecular mechanism during oocyte maturation, especially epigenetic modification, the progress of the regulatory network of oocyte meiosis progression including meiosis arrest and meiosis resumption induced by gonadotropins is summarized. Then, advances in the epigenetic aspects, such as histone acetylation, phosphorylation, methylation, glycosylation, ubiquitination, and SUMOylation related to the quality of oocyte maturation are reviewed.

Dissection of the Ovulatory Process Using ex vivo Approaches

Ovulation is a unique physiological phenomenon that is essential for sexual reproduction. It refers to the entire process of ovarian follicle responses to hormonal stimulation resulting in the release of mature fertilization-competent oocytes from the follicles and ovaries. Remarkably, ovulation in different species can be reproduced out-of-body with high fidelity. Moreover, most of the molecular mechanisms and signaling pathways engaged in this process have been delineated using in vitro ovulation models. Here, we provide an overview of the major molecular and cytological events of ovulation observed in frogs, primarily in the African clawed frog Xenopus laevis, using mainly ex vivo approaches, with the focus on meiotic oocyte maturation and follicle rupture. For the purpose of comparison and generalization, we also refer extensively to ovulation in other biological species, most notoriously, in mammals.

A G protein-coupled receptor mediates neuropeptide-induced oocyte maturation in the jellyfish Clytia

The reproductive hormones that trigger oocyte meiotic maturation and release from the ovary vary greatly between animal species. Identification of receptors for these maturation-inducing hormones (MIHs) and understanding how they initiate the largely conserved maturation process remain important challenges. In hydrozoan cnidarians including the jellyfish Clytia hemisphaerica, MIH comprises neuropeptides released from somatic cells of the gonad. We identified the receptor (MIHR) for these MIH neuropeptides in Clytia using cell culture–based “deorphanization” of candidate oocyte-expressed G protein–coupled receptors (GPCRs). MIHR mutant jellyfish generated using CRISPR-Cas9 editing had severe defects in gamete development or in spawning both in males and females. Female gonads, or oocytes isolated from MIHR mutants, failed to respond to synthetic MIH. Treatment with the cAMP analogue Br-cAMP to mimic cAMP rise at maturation onset rescued meiotic maturation and spawning. Injection of inhibitory antibodies to the alpha subunit of the G_s heterodimeric protein (Gα_S) into wild-type oocytes phenocopied the MIHR mutants. These results provide the molecular links between MIH stimulation and meiotic maturation initiation in hydrozoan oocytes. Molecular phylogeny grouped Clytia MIHR with a subset of bilaterian neuropeptide receptors, including neuropeptide Y, gonadotropin inhibitory hormone (GnIH), pyroglutamylated RFamide, and luqin, all upstream regulators of sexual reproduction. This identification and functional characterization of a cnidarian peptide GPCR advances our understanding of oocyte maturation initiation and sheds light on the evolution of neuropeptide-hormone systems.

A study of jellyfish oocytes identifies the receptor for Maturation-Inducing Hormone, the neuropeptide hormone that triggers oocyte maturation and spawning via GαS and cyclic AMP. This receptor defines a superfamily of hormone-receptor systems involved in regulating sexual reproduction across animal species.

Implication of mu opioid receptor in the in vitro maturation of oocytes and its effects on subsequent fertilization and embryo development in mice

Oocyte maturation is the process by which immature oocytes acquire all the necessary characteristics for successful fertilization. The endogenous opioid peptides have been suggested to have a role modulating this process. However, little is known about its implication and the effect of exposing oocyte maturation to opioids on the subsequent fertilization and embryo development. Hence, in the present work, we focused on elucidating the function of the mu opioid receptor (OPRM1) in the modulation of the oocyte maturation. We analyzed the expression and localization of OPRM1 in mice oocytes and granulosa cells by reverse-transcription polymerase chain reaction (RT-PCR) and immunocytochemistry. To observe the activity of the OPRM1, immature oocytes were incubated with morphine agonist and/or naloxone antagonist and we evaluated the PI3K/Akt and MAPK pathways, as well as the effect on the subsequent fertilization and embryo development. OPRM1 was present in mice oocytes and granulosa cells, changing its expression pattern depending on the maturation stage. Moreover, morphine, modulating PI3K/Akt and MAPK pathways, helped oocytes to reach blastocyst stage, which was reverted by naloxone. These results propose the OPRM1 as a possible therapeutic target for in vitro maturation culture medium, as it could improve the blastocyst rates obtained in the actual reproduction assisted techniques.

The art of oocyte meiotic arrest regulation

A central dogma of mammalian reproductive biology is that the size of the primordial follicle pool represents reproductive capacity in females. The assembly of the primordial follicle starts after the primordial germ cells (PGCs)-derived oocyte releases from the synchronously dividing germline cysts. PGCs initiate meiosis during fetal development. However, after synapsis and recombination of homologous chromosomes, they arrest at the diplotene stage of the first meiotic prophase (MI). The diplotene-arrested oocyte, together with the surrounding of a single layer of flattened granulosa cells, forms a basic unit of the ovary, the primordial follicle. At the start of each estrous (animal) or menstrual cycle (human), in response to a surge of luteinizing hormone (LH) from the pituitary gland, a limited number of primordial follicles are triggered to develop into primary follicles, preantral follicles, antral follicles and reach to preovulatory follicle stage. During the transition from the preantral to antral stages, the enclosed oocyte gradually acquires the capacity to resume meiosis. Meiotic resumption from the prophase of MI is morphologically characterized by the dissolution of the oocyte nuclear envelope, which is generally termed the "germinal vesicle breakdown" (GVBD). Following GVBD and completion of MI, the oocyte enters meiosis II without an obvious S-phase and arrests at metaphase phase II (MII) until fertilization. The underlying mechanism of meiotic arrest has been widely explored in numerous studies. Many studies indicated that two cellular second messengers, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) play an essential role in maintaining oocyte meiotic arrest. This review will discuss how these two cyclic nucleotides regulate oocyte maturation by blocking or initiating meiotic processes, and to provide an insight in future research.

Oocyte-specific deletion of Gsα induces oxidative stress and deteriorates oocyte quality in mice

The stimulatory heterotrimeric G_s protein alpha subunit (G_sα) is a ubiquitous guanine nucleotide-binding protein that regulates the intracellular cAMP signaling pathway and consequently participates in a wide range of biological events. In the reproductive system, despite G_sα being associated with oocyte meiotic arrest in vitro, the exact role of G_sα in female fertility in vivo remains largely unknown. Here, we generated oocyte-specific G_sα knockout mice by using the Cre/LoxP system. We observed that the deletion of G_sα caused complete female infertility. Exclusion of post-implantation abnormalities, oogenesis, fertilization, and early embryo development was subsequently monitored; meiosis in G_sα-deficient oocytes precociously resumed in only 43% of antral follicles from mutant mice, indicating that alteration of meiotic pause was not the key factor in infertility. Ovulation process and number were normal, but the rate of morphological abnormal oocytes was apparently increased; spindle organization, fertilization, and early embryo development were impaired. Furthermore, the level of ROS (reactive oxygen species) and the mitochondrial aggregation increased, and antioxidant glutathione (GSH) content, ATP level, mtDNA copy number, and mitochondrial membrane potential decreased in G_sα-deficient oocytes. GV oocytes from mutant mice showed early-stage apoptosis. Meanwhile, the G_sα knockout-induced decline in oocyte quality and low developmental potential was partially rescued by antioxidant supplementation. To sum up, our results are the first to reveal that the profile of G_sα oocyte-specific deletion caused female infertility in vivo, and oxidative stress plays an important role in this event.

Endocrine, Paracrine, and Autocrine Signaling Pathways That Regulate Ovulation

The central role of luteinizing hormone (LH) and its receptor (LHCGR) in triggering ovulation has been recognized for decades. Because the LHCGR is present in the mural (outermost) granulosa cell layer of preovulatory follicles (POFs), the LH-initiated signal has to be transmitted to another somatic cell type (cumulus granulosa cells) and the oocyte to release a fertilizable oocyte. Recent studies have shown that activation of the LHCGR initiates vectorial transfer of information among the two somatic cell types and the oocyte and the molecules and signaling pathways involved are now better understood. This review summarizes the newer developments on the complex signaling pathways that regulate ovulation.

Acquisition of oocyte competence to develop as an embryo: integrated nuclear and cytoplasmic events

Infertility affects ~7% of couples of reproductive age with little change in incidence in the last two decades. ART, as well as other interventions, have made major strides in correcting this condition. However, and in spite of advancements in the field, the age of the female partner remains a main factor for a successful outcome. A better understanding of the final stages of gamete maturation yielding an egg that can sustain embryo development and a pregnancy to term remains a major area for improvement in the field. This review will summarize the major cellular and molecular events unfolding at the oocyte-to-embryo transition. We will provide an update on the most important processes/pathways currently understood as the basis of developmental competence, including the molecular processes involved in mRNA storage, its recruitment to the translational machinery, and its degradation. We will discuss the hypothesis that the translational programme of maternal mRNAs plays a key role in establishing developmental competence. These regulations are essential to assemble the machinery that is used to establish a totipotent zygote. This hypothesis further supports the view that embryogenesis begins during oogenesis. A better understanding of the events required for developmental competence will guide the development of novel strategies to monitor and improve the success rate of IVF. Using this information, it will be possible to develop new biomarkers that may be used to better predict oocyte quality and in selection of the best egg for IVF.

Towards a better understanding of the cannabinoid-related orphan receptors GPR3, GPR6, and GPR12

GPR3, GPR6, and GPR12 are three orphan receptors that belong to the Class A family of G-protein-coupled receptors (GPCRs). These GPCRs share over 60% of sequence similarity among them. Because of their close phylogenetic relationship, GPR3, GPR6, and GPR12 share a high percentage of homology with other lipid receptors such as the lysophospholipid and the cannabinoid receptors. On the basis of sequence similarities at key structural motifs, these orphan receptors have been related to the cannabinoid family. However, further experimental data are required to confirm this association. GPR3, GPR6, and GPR12 are predominantly expressed in mammalian brain. Their high constitutive activation of adenylyl cyclase triggers increases in cAMP levels similar in amplitude to fully activated GPCRs. This feature defines their physiological role under certain pathological conditions. In this review, we aim to summarize the knowledge attained so far on the understanding of these receptors. Expression patterns, pharmacology, physiopathological relevance, and molecules targeting GPR3, GPR6, and GPR12 will be analyzed herein. Interestingly, certain cannabinoid ligands have been reported to modulate these orphan receptors. The current debate about sphingolipids as putative endogenous ligands will also be addressed. A special focus will be on their potential role in the brain, particularly under neurological conditions such as Parkinson or Alzheimer's disease. Reported physiological roles outside the central nervous system will also be covered. This critical overview may contribute to a further comprehension of the physiopathological role of these orphan GPCRs, hopefully attracting more research towards a future therapeutic exploitation of these promising targets.

[Signaling and functions of G-protein-coupled receptor 3 in cerebellar granular neurons]

G-protein-coupled receptor 3 (GPR3) is a member of the class A rhodopsin-type GPCR family and is highly expressed in various neurons. A unique feature of GPR3 is its ability to constitutively activate the Gαs protein without the addition of ligands, which results in the elevation of the basal level of intracellular cAMP. During the development of the cerebellum, GPR3 expression is upregulated in cerebellar granular neurons (CGNs) and maintained thereafter. In our previous studies, we showed that the intrinsic expression of GPR3 in CGNs is highly associated with neurite outgrowth, neurite differentiation, and neuronal survival. Recently, we have focused on the possible signaling pathways associated with GPR3-mediated neurite outgrowth in CGNs. Interestingly, GPR3-mediated neurite outgrowth is mediated by not only PKA-dependent signaling pathways but also PI3K-mediated signaling pathways. Moreover, the Gβγ-mediated signaling pathway is involved in GPR3-mediated neurite outgrowth. These results suggested that neural expression of GPR3 stimulates multiple downstream signaling pathways, contributing to the maintenance of homeostasis in neurons. Further precise analyses of constitutively active GPCRs may help in unveiling novel neuronal functions.

VLDL receptor regulates membrane progesterone receptor trafficking and non-genomic signaling

Progesterone mediates its physiological functions through activation of both transcription-coupled nuclear receptors and 7-transmembrane progesterone receptors (mPRs) that transduce progesterone's rapid non-genomic actions by coupling to various signaling modules. However, the immediate mechanisms of action downstream of mPRs remain in question. Herein we use an untargeted quantitative proteomics approach to identify mPR interactors to better define progesterone non-genomic signaling. Surprisingly, we identify the VLDL Receptor (VLDLR) as an mPR partner required for its plasma membrane localization. Knocking down VLDLR abolishes non-genomic progesterone signaling, a phenotype that is rescued by overexpressing VLDLR. Mechanistically, we show that the VLDLR is required for mPR trafficking from the ER to the Golgi. Taken together, our data define a novel function for the VLDLR as a trafficking chaperone required for the mPR subcellular localization and as such non-genomic progesterone-dependent signaling.

The switch from cAMP-independent to cAMP-dependent arrest of meiotic prophase is associated with coordinated GPR3 and CDK1 expression in mouse oocytes

Mammalian oocytes are arrested in meiotic prophase from around the time of birth until just before ovulation. Following an extended period of growth, they are stimulated to mature to the metaphase II stage by a preovulatory luteinizing hormone (LH) surge that occurs with each reproductive cycle. Small, growing oocytes are not competent to mature into fertilizable eggs because they do not possess adequate amounts of cell cycle regulatory proteins, particularly cyclin-dependent kinase 1 (CDK1). As oocytes grow, they synthesize CDK1 and acquire the ability to mature. After oocytes achieve meiotic competence, meiotic arrest at the prophase stage is dependent on high levels of cAMP that are generated in the oocyte under the control of the constitutively active G_s-coupled receptor, GPR3. In this study, we examined the switch between GPR3-independent and GPR3-dependent meiotic arrest. We found that the ability of oocytes to mature, as well as oocyte CDK1 levels, were dependent on follicle size, but CDK1 expression in oocytes from preantral follicles was not acutely altered by the activity of follicle stimulating hormone (FSH). Gpr3 was expressed and active in incompetent oocytes within early stage follicles, well before cAMP is required to maintain meiotic arrest. Oocytes from Gpr3^-/- mice were less competent to mature than oocytes from Gpr3^+/+ mice, as assessed by the time course of germinal vesicle breakdown. Correspondingly, Gpr3^-/- oocytes contained significantly lower CDK1 levels than their Gpr3^+/+ counterparts that were at the same stage of follicle development. These results demonstrate that GPR3 potentiates meiotic competence, most likely by raising cAMP.

The role of L-type calcium channels in mouse oocyte maturation, activation and early embryonic development

Calcium ion fluctuation is closely related to the transformation of cell cycle. However, little is known about the function of L-type calcium channel in mammalian oocyte and embryo development. We thus studied the roles of L-type calcium channel in mouse oocyte meiotic maturation, parthenogenetic activation and early embryonic development. We used the antagonist Amlodipine to block L-type calcium channel. Oocytes or zygotes were cultured to different time points with 0 μM, 10 μM, 30 μM and 50 μM Amlodipine. Then we checked the rate of first polar body extrusion, spindle formation, asymmetric division parthenogenetic activation and early embryo cleavage. The results showed that Amlodipine treatment did not affect germinal vesicle breakdown, but caused disruption of cytoskeleton organization, symmetric division, formation of mature oocytes with a large polar body, or reduced the first polar body extrusion, depending on its concentrations. Amlodipine treatment also resulted in decreased parthenogenetic activation and arrested early embryonic development. Overall, these data suggest that proper function of L-type calcium channel is critical for oocyte maturation, activation, and early embryonic development.

CB1 cannabinoid receptor drives oocyte maturation and embryo development via PI3K/Akt and MAPK pathways

Endocannabinoids have been recognized as mediators of practically all reproductive events in mammals. However, little is known about the role of this system in oocyte maturation. In a mouse model, we observed that activation of cannabinoid receptor 1 (CB₁) during in vitro oocyte maturation modulated the phosphorylation status of Akt and ERK1/2 and enhanced the subsequent embryo production. In the absence of CB₁, in vivo oocyte maturation was impaired and embryo development delayed. Cannabinoid receptor 2 (CB₂) was unable to rescue these effects. Finally, we confirmed abnormal oocyte maturation rather than impaired embryonic transport through the oviduct in CB₁ knockouts. Our data suggest that cannabinoid agonists may be useful in vitro maturation supplements. For in vitro fertilization patients intolerant to gonadotropins, this could be a promising and only option.-López-Cardona, A. P., Pérez-Cerezales, S., Fernández-González, R., Laguna-Barraza, R., Pericuesta, E., Agirregoitia, N., Gutiérrez-Adán, A., Agirregoitia, E. CB₁ cannabinoid receptor drives oocyte maturation and embryo development via PI3K/Akt and MAPK pathways.

Regulation of Mammalian Oocyte Meiosis by Intercellular Communication Within the Ovarian Follicle

Meiotic progression in mammalian preovulatory follicles is controlled by the granulosa cells around the oocyte. Cyclic GMP (cGMP) generated in the granulosa cells diffuses through gap junctions into the oocyte, maintaining meiotic prophase arrest. Luteinizing hormone then acts on receptors in outer granulosa cells to rapidly decrease cGMP. This occurs by two complementary pathways: cGMP production is decreased by dephosphorylation and inactivation of the NPR2 guanylyl cyclase, and cGMP hydrolysis is increased by activation of the PDE5 phosphodiesterase. The cGMP decrease in the granulosa cells results in rapid cGMP diffusion out of the oocyte, initiating meiotic resumption. Additional, more slowly developing mechanisms involving paracrine signaling by extracellular peptides (C-type natriuretic peptide and EGF receptor ligands) maintain the low level of cGMP in the oocyte. These coordinated signaling pathways ensure a fail-safe system to prepare the oocyte for fertilization and reproductive success.

Release from Xenopus oocyte prophase I meiotic arrest is independent of a decrease in cAMP levels or PKA activity

Vertebrate oocytes arrest at prophase of meiosis I as a result of high levels of cyclic adenosine monophosphate (cAMP) and protein kinase A (PKA) activity. In Xenopus, progesterone is believed to release meiotic arrest by inhibiting adenylate cyclase, lowering cAMP levels and repressing PKA. However, the exact timing and extent of the cAMP decrease is unclear, with conflicting reports in the literature. Using various in vivo reporters for cAMP and PKA at the single-cell level in real time, we fail to detect any significant changes in cAMP or PKA in response to progesterone. More interestingly, there was no correlation between the levels of PKA inhibition and the release of meiotic arrest. Furthermore, we devised conditions whereby meiotic arrest could be released in the presence of sustained high levels of cAMP. Consistently, lowering endogenous cAMP levels by >65% for prolonged time periods failed to induce spontaneous maturation. These results argue that the release of oocyte meiotic arrest in Xenopus is independent of a reduction in either cAMP levels or PKA activity, but rather proceeds through a parallel cAMP/PKA-independent pathway.

Multiple Mechanisms Cooperate to Constitutively Exclude the Transcriptional Co-Activator YAP from the Nucleus During Murine Oogenesis

Reproduction depends on the generation of healthy oocytes. Improving therapeutic strategies to prolong or rescue fertility depends on identifying the inter- and intracellular mechanisms that direct oocyte development under physiological conditions. Growth and proliferation of multiple cell types is regulated by the Hippo signaling pathway, whose chief effectors are the transcriptional co-activator YAP and its paralogue WWTR1. To resolve conflicting results concerning the potential role of Hippo in mammalian oocyte development, we systematically investigated the expression and localization of YAP in mouse oocytes. We report that that YAP is expressed in the germ cells beginning as early as Embryonic Day 15.5 and subsequently throughout pre- and postnatal oocyte development. However, YAP is restricted to the cytoplasm at all stages. YAP is phosphorylated at serine-112 in growing and fully grown oocytes, identifying a likely mechanistic basis for its nuclear exclusion, and becomes dephosphorylated at this site during meiotic maturation. Phosphorylation at serine-112 is regulated by a mechanism dependent on cyclic AMP and protein kinase A, which is known to be active in oocytes prior to maturation. Growing oocytes also contain a subpopulation of YAP, likely dephosphorylated, that is able enter the oocyte nucleus, but it is not retained there, implying that oocytes lack the cofactors required to retain YAP in the nucleus. Thus, although YAP is expressed throughout oocyte development, phosphorylation-dependent and -independent mechanisms cooperate to ensure that it does not accumulate in the nucleus. We conclude that nuclear YAP does not play a significant physiological role during oocyte development in mammals.

The Subcellular Dynamics of the Gs-Linked Receptor GPR3 Contribute to the Local Activation of PKA in Cerebellar Granular Neurons

G-protein-coupled receptor (GPR) 3 is a member of the GPR family that constitutively activates adenylate cyclase. We have reported that the expression of GPR3 in cerebellar granular neurons (CGNs) contributes to neurite outgrowth and modulates neuronal proliferation and survival. To further identify its role, we have analyzed the precise distribution and local functions of GPR3 in neurons. The fluorescently tagged GPR3 protein was distributed in the plasma membrane, the Golgi body, and the endosomes. In addition, we have revealed that the plasma membrane expression of GPR3 functionally up-regulated the levels of PKA, as measured by a PKA FRET indicator. Next, we asked if the PKA activity was modulated by the expression of GPR3 in CGNs. PKA activity was highly modulated at the neurite tips compared to the soma. In addition, the PKA activity at the neurite tips was up-regulated when GPR3 was transfected into the cells. However, local PKA activity was decreased when endogenous GPR3 was suppressed by a GPR3 siRNA. Finally, we determined the local dynamics of GPR3 in CGNs using time-lapse analysis. Surprisingly, the fluorescent GPR3 puncta were transported along the neurite in both directions over time. In addition, the anterograde movements of the GPR3 puncta in the neurite were significantly inhibited by actin or microtubule polymerization inhibitors and were also disturbed by the Myosin II inhibitor blebbistatin. Moreover, the PKA activity at the tips of the neurites was decreased when blebbistatin was administered. These results suggested that GPR3 was transported along the neurite and contributed to the local activation of PKA in CGN development. The local dynamics of GPR3 in CGNs may affect local neuronal functions, including neuronal differentiation and maturation.

Mice lacking GPR3 receptors display late-onset obese phenotype due to impaired thermogenic function in brown adipose tissue

We report an unexpected link between aging, thermogenesis and weight gain via the orphan G protein-coupled receptor GPR3. Mice lacking GPR3 and maintained on normal chow had similar body weights during their first 5 months of life, but gained considerably more weight thereafter and displayed reduced total energy expenditure and lower core body temperature. By the age of 5 months GPR3 KO mice already had lower thermogenic gene expression and uncoupling protein 1 protein level and showed impaired glucose uptake into interscapular brown adipose tissue (iBAT) relative to WT littermates. These molecular deviations in iBAT of GPR3 KO mice preceded measurable differences in body weight and core body temperature at ambient conditions, but were coupled to a failure to maintain thermal homeostasis during acute cold challenge. At the same time, the same cold challenge caused a 17-fold increase in Gpr3 expression in iBAT of WT mice. Thus, GPR3 appears to have a key role in the thermogenic response of iBAT and may represent a new therapeutic target in age-related obesity.

Vertebrate Reproduction

Vertebrate reproduction requires a myriad of precisely orchestrated events-in particular, the maternal production of oocytes, the paternal production of sperm, successful fertilization, and initiation of early embryonic cell divisions. These processes are governed by a host of signaling pathways. Protein kinase and phosphatase signaling pathways involving Mos, CDK1, RSK, and PP2A regulate meiosis during maturation of the oocyte. Steroid signals-specifically testosterone-regulate spermatogenesis, as does signaling by G-protein-coupled hormone receptors. Finally, calcium signaling is essential for both sperm motility and fertilization. Altogether, this signaling symphony ensures the production of viable offspring, offering a chance of genetic immortality.

Oocyte maturation: gamete-somatic cells interactions, meiotic resumption, cytoskeletal dynamics and cytoplasmic reorganization

BACKGROUND

In a growth phase occurring during most of folliculogenesis, the oocyte produces and accumulates molecules and organelles that are fundamental for the development of the preimplantation embryo. At ovulation, growth is followed by a phase of maturation that, although confined within a short temporal window, encompasses modifications of the oocyte chromosome complement and rearrangements of cytoplasmic components that are crucial for the achievement of developmental competence. Cumulus cells (CCs) are central to the process of maturation, providing the oocyte with metabolic support and regulatory cues.

METHODS

PubMed was used to search the MEDLINE database for peer-reviewed original articles and reviews concerning oocyte maturation in mammals. Searches were performed adopting 'oocyte' and 'maturation' as main terms, in association with other keywords expressing concepts relevant to the subject. The most relevant publications, i.e. those concerning major phenomena occurring during oocyte maturation in established experimental models and the human species, were assessed and discussed critically to offer a comprehensive description of the process of oocyte maturation.

RESULTS

By applying the above described search criteria, 6165 publications were identified, of which 543 were review articles. The number of publications increased steadily from 1974 (n = 7) to 2013 (n = 293). In 2014, from January to the time of submission of this manuscript, 140 original manuscripts and reviews were published. The studies selected for this review extend previous knowledge and shed new and astounding knowledge on oocyte maturation. It has long been known that resumption of meiosis and progression to the metaphase II stage is intrinsic to oocyte maturation, but novel findings have revealed that specific chromatin configurations are indicative of a propensity of the oocyte to resume the meiotic process and acquire developmental competence. Recently, genetic integrity has also been characterized as a factor with important implications for oocyte maturation and quality. Changes occurring in the cytoplasmic compartment are equally fundamental. Microtubules, actin filaments and chromatin not only interact to finalize chromosome segregation, but also crucially co-operate to establish cell asymmetry. This allows polar body extrusion to be accomplished with minimal loss of cytoplasm. The cytoskeleton also orchestrates the rearrangement of organelles in preparation for fertilization. For example, during maturation the distribution of the endoplasmic reticulum undergoes major modifications guided by microtubules and microfilaments to make the oocyte more competent in the generation of intracellular Ca(2+) oscillations that are pivotal for triggering egg activation. Cumulus cells are inherent to the process of oocyte maturation, emitting regulatory signals via direct cell-to-cell contacts and paracrine factors. In addition to nurturing the oocyte with key metabolites, CCs regulate meiotic resumption and modulate the function of the oocyte cytoskeleton.

CONCLUSIONS

Although the importance of oocyte maturation for the achievement of female meiosis has long been recognized, until recently much less was known of the significance of this process in relation to other fundamental developmental events. Studies on chromatin dynamics and integrity have extended our understanding of female meiosis. Concomitantly, cytoskeletal and organelle changes and the ancillary role of CCs have been better appreciated. This is expected to inspire novel concepts and advances in assisted reproduction technologies, such as the development of novel in vitro maturation systems and the identification of biomarkers of oocyte quality.

Redistribution of Gαs in mouse salivary glands following β-adrenergic stimulation

OBJECTIVE

Signalling via β-adrenergic receptors activates heterotrimeric G-proteins, which dissociate into α and βγ subunits. In salivary glands, the α subunit of Gs stimulates adenylate cyclase, increasing cyclic AMP levels and promoting exocytosis. The goals of this study were to determine Gαs localization in salivary glands and whether it undergoes redistribution upon activation.

METHODS

Mouse parotid and submandibular (SMG) glands were fixed with paraformaldehyde and prepared for immunofluorescence labelling with anti-Gαs.

RESULTS

In unstimulated parotid and SMG acinar cells, Gαs was localized mainly to basolateral membranes. Some parotid acinar cells also exhibited cytoplasmic fluorescence. Isoproterenol (IPR) stimulation resulted in decreased membrane fluorescence and increased cytoplasmic fluorescence, which appeared relatively uniform by 30 min. Beginning about 2 h after IPR, cytoplasmic fluorescence decreased and membrane fluorescence increased, approaching unstimulated levels in SMG acini by 4 h. Some parotid acini exhibited cytoplasmic fluorescence up to 8 h after IPR. The IPR-induced redistribution of Gαs was prevented (SMG) or reduced (parotid) by prior injection of propranolol. Striated duct cells of unstimulated mice exhibited general cytoplasmic fluorescence, which was unchanged after IPR.

CONCLUSIONS

Gαs is localized to basolateral membranes of unstimulated salivary acinar cells. Activation of Gαs causes its release from the cell membrane and movement into the cytoplasm. Reassociation of Gαs with the membrane begins about 2 h after stimulation in the SMG, but complete reassociation takes several hours in the parotid gland. The presence of Gαs in striated duct cells suggests a role in signal transduction of secretion and/or electrolyte transport processes.

Intercellular Lipid Mediators and GPCR Drug Discovery

G-protein-coupled receptors (GPCR) are the largest superfamily of receptors responsible for signaling between cells and tissues, and because they play important physiological roles in homeostasis, they are major drug targets. New technologies have been developed for the identification of new ligands, new GPCR functions, and for drug discovery purposes. In particular, intercellular lipid mediators, such as, lysophosphatidic acid and sphingosine 1-phosphate have attracted much attention for drug discovery and this has resulted in the development of fingolimod (FTY-720) and AM095. The discovery of new intercellular lipid mediators and their GPCRs are discussed from the perspective of drug development. Lipid GPCRs for lysophospholipids, including lysophosphatidylserine, lysophosphatidylinositol, lysophosphatidylcholine, free fatty acids, fatty acid derivatives, and other lipid mediators are reviewed.

Transducin-like enhancer of split-6 (TLE6) is a substrate of protein kinase A activity during mouse oocyte maturation

Fully grown oocytes in the ovary are arrested at prophase of meiosis I because of high levels of intraoocyte cAMP that maintain increased levels of cAMP-dependent protein kinase (PKA) activity. Following the luteinizing hormone surge at the time of ovulation, cAMP levels drop, resulting in a reduction in PKA activity that triggers meiotic resumption. Although much is known about the molecular mechanisms of how PKA activity fluctuations initiate the oocyte's reentry into meiosis, significantly less is known about the requirement for PKA activity in the oocyte after exit from the prophase I arrest. Here we show that although PKA activity decreases in the oocyte upon meiotic resumption, it increases throughout meiotic progression from the time of germinal vesicle breakdown (GVBD) until the metaphase II (MII) arrest. Blocking this meiotic maturation-associated increase in PKA activity using the pharmacological inhibitor H89 resulted in altered kinetics of GVBD, defects in chromatin and spindle dynamics, and decreased ability of oocytes to reach MII. These effects appear to be largely PKA specific because inhibitors targeting other kinases did not have the same outcomes. To determine potential proteins that may require PKA phosphorylation during meiosis, we separated oocyte protein extracts on an SDS-PAGE gel, extracted regions of the gel that had corresponding immune reactivity towards an anti-PKA substrate antibody, and performed mass spectrometry and microsequencing. Using this approach, we identified transducin-like enhancer of split-6 (TLE6)-a maternal effect gene that is part of the subcortical maternal complex-as a putative PKA substrate. TLE6 localized to the oocyte cortex throughout meiosis in a manner that is spatially and temporally consistent with the localization of critical PKA subunits. Moreover, we demonstrated that TLE6 becomes phosphorylated in a narrow window following meiotic resumption, and H89 treatment can completely block this phosphorylation when added prior to GVBD but not after. Taken together, these results highlight the importance of oocyte-intrinsic PKA in regulating meiotic progression after the prophase I arrest and offer new insights into downstream targets of its activity.

Regulation of Constitutive GPR3 Signaling and Surface Localization by GRK2 and β-arrestin-2 Overexpression in HEK293 Cells

G protein-coupled receptor 3 (GPR3) is a constitutively active receptor that maintains high 3′-5′-cyclic adenosine monophosphate (cAMP) levels required for meiotic arrest in oocytes and CNS function. Ligand-activated G protein-coupled receptors (GPCRs) signal at the cell surface and are silenced by phosphorylation and β-arrestin recruitment upon endocytosis. Some GPCRs can also signal from endosomes following internalization. Little is known about the localization, signaling, and regulation of constitutively active GPCRs. We demonstrate herein that exogenously-expressed GPR3 localizes to the cell membrane and undergoes internalization in HEK293 cells. Inhibition of endocytosis increased cell surface-localized GPR3 and cAMP levels while overexpression of GPCR-Kinase 2 (GRK2) and β-arrestin-2 decreased cell surface-localized GPR3 and cAMP levels. GRK2 by itself is sufficient to decrease cAMP production but both GRK2 and β-arrestin-2 are required to decrease cell surface GPR3. GRK2 regulates GPR3 independently of its kinase activity since a kinase inactive GRK2-K220R mutant significantly decreased cAMP levels. However, GRK2-K220R and β-arrestin-2 do not diminish cell surface GPR3, suggesting that phosphorylation is required to induce GPR3 internalization. To understand which residues are targeted for desensitization, we mutated potential phosphorylation sites in the third intracellular loop and C-terminus and examined the effect on cAMP and receptor surface localization. Mutation of residues in the third intracellular loop dramatically increased cAMP levels whereas mutation of residues in the C-terminus produced cAMP levels comparable to GPR3 wild type. Interestingly, both mutations significantly reduced cell surface expression of GPR3. These results demonstrate that GPR3 signals at the plasma membrane and can be silenced by GRK2/β-arrestin overexpression. These results also strongly implicate the serine and/or threonine residues in the third intracellular loop in the regulation of GPR3 activity.

The orphan receptor GPR3 modulates the early phases of cocaine reinforcement

BACKGROUND AND PURPOSE

The modulatory activity of the orphan receptor GPR3 in the brain has been related to the control of emotional behaviours. Limbic structures that express GPR3 have been associated with the effects of drug abuse.

EXPERIMENTAL APPROACH

The role of GPR3 in different cocaine-elicited behaviours including locomotor activity, behavioural sensitization, conditioned place preference (CPP) and intravenous self-administration was evaluated in Gpr3-/- mice and their Gpr3+/+ littermates. Cocaine-induced dopamine release in the nucleus accumbens was also evaluated to elucidate the effect of Gpr3 deletion on extracellular levels of dopamine.

KEY RESULTS

Gpr3-/- mice exhibited higher rewarding responses in the CPP paradigm. Gpr3-/- mice self-administered more cocaine, especially during the first days of training. No differences were found between genotypes regarding behavioural sensitization and the maximal effort required to obtain a cocaine infusion. Non-contingent priming injections of cocaine before operant training eliminated enhanced cocaine self-administration in Gpr3-/- mice. Extracellular levels of dopamine in the nucleus accumbens induced by cocaine did not differ between genotypes.

CONCLUSIONS AND IMPLICATIONS

The increased responsiveness of Gpr3-/- mice to the acute locomotor effects of cocaine and the inconsistency to further increase this effect reflected an 'already maximally sensitized' basal state. Enhanced responsiveness of Gpr3-/- mice to cocaine reward and to early phases of reinforcement suggests that an initial alteration increased vulnerability to this type of drug abuse. Overall, altered signalling pathways of GPR3 could contribute to the neurobiological substrate involved in developing addiction to cocaine.

FSH modulates PKAI and GPR3 activities in mouse oocyte of COC in a gap junctional communication (GJC)-dependent manner to initiate meiotic resumption

Many studies have shown that cyclic adenosine-5′-monophosphate (cAMP)-dependent protein kinase A (PKA) and G-protein-coupled receptor 3 (GPR3) are crucial for controlling meiotic arrest in oocytes. However, it is unclear how gonadotropins modulate these factors to regulate oocyte maturation, especially by gap junctional communication (GJC). Using an in vitro meiosis-arrested mouse cumulus-oocyte complex (COC) culture model, we showed that there is a close relationship between follicle-stimulating hormone (FSH) and the PKA type I (PKAI) and GPR3. The effect of FSH on oocyte maturation was biphasic, initially inhibitory and then stimulatory. During FSH-induced maturation, rapid cAMP surges were observed in both cumulus cells and oocyte. Most GJC between cumulus cells and oocyte ceased immediately after FSH stimulation and recommenced after the cAMP surge. FSH-induced maturation was blocked by PKAI activator 8-AHA-cAMP. Levels of PKAI regulatory subunits and GPR3 decreased and increased, respectively, after FSH stimulation. In the presence of the GJC inhibitor carbenoxolone (CBX), FSH failed to induce the meiotic resumption and the changes in PKAI, GPR3 and cAMP surge in oocyte were no longer detected. Furthermore, GPR3 was upregulated by high cAMP levels, but not by PKAI activation. When applied after FSH stimulation, the specific phosphodiesterase 3A (PDE3A) inhibitor cilostamide immediately blocked meiotic induction, regardless of when it was administered. PKAI activation inhibited mitogen-activated protein kinase (MAPK) phosphorylation in the oocytes of COCs, which participated in the initiation of FSH-induced meiotic maturation in vitro. Just before FSH-induced meiotic maturation, cAMP, PKAI, and GPR3 returned to basal levels, and PDE3A activity and MAPK phosphorylation increased markedly. These experiments show that FSH induces a transient increase in cAMP levels and regulates GJC to control PKAI and GPR3 activities, thereby creating an inhibitory phase. After PDE3A and MAPK activities increase, meiosis resumes.

Distribution of adenylyl cyclases in the rat ovary by immunofluorescence microscopy

In the mammalian ovary both gonadotropins and local cytokines, acting through G-protein coupled receptors, govern the physiology of the ovary in part by regulating the cyclic adenosine 3',5'-monophosphate via adenylyl cyclases. The nine transmembrane adenylyl cyclases and a soluble adenylyl cyclase are regulated by a diversity of ligands. In this study we have examined the rat ovaries, prior to and subsequent to gonadotropin treatment, for the presence of different transmembrane adenylyl cyclases by indirect immunofluorescence microscopy. Adenylyl cyclase I immunoreactivity was observed in the nuclei of oocytes in preantral and antral follicles along with some staining in granulosa cells. Equine chorionic gonadotropin injection increased adenylyl cyclase I staining in granulosa cells. Adenylyl cyclase I staining was also observed in luteal and endothelial cells. Adenylyl cyclase II was observed throughout the ovary, including granulosa cells and the ovarian surface epithelium. Adenylyl cyclase II staining was also found to increase in granulosa cells after equine chorionic gonadotropin injection. Adenylyl cyclase III was distributed primarily in theca and smooth muscle cells of arterioles, with faint staining in the oocytes of equine chorionic gonadotropin-injected ovaries. Adenylyl cyclase IV staining was present throughout the ovary, including the nuclei of oocytes. Adenylyl cyclase VIII staining in granulosa cells increased subsequent to equine chorionic gonadotropin injection and remained in luteal cells. Our study reveals the redundancy of adenylyl cyclases present in the rat ovary and, therefore, implies potential regulation of follicular and corpus luteum physiology by cyclic adenosine 3',5'-monophosphate generated through distinct adenylyl cyclases.

The G protein coupled receptor 3 is involved in cAMP and cGMP signaling and maintenance of meiotic arrest in porcine oocytes

The arrest of meiotic prophase in mammalian oocytes within fully grown follicles is dependent on cyclic adenosine monophosphate (cAMP) regulation. A large part of cAMP is produced by the Gs-linked G-protein-coupled receptor (GPR) pathway. In the present study, we examined whether GPR3 is involved in the maintenance of meiotic arrest in porcine oocytes. Expression and distribution of GPR3 were examined by western blot and immunofluorescence microscopy, respectively. The results showed that GPR3 was expressed at various stages during porcine oocyte maturation. At the germinal vesicle (GV) stage, GPR3 displayed a maximal expression level, and its expression remained stable from pro-metaphase I (MI) to metaphase II (MII). Immunofluorescence staining showed that GPR3 was mainly distributed at the nuclear envelope during the GV stage and localized to the plasma membrane at pro-MI, MI and MII stages. RNA interference (RNAi) was used to knock down the GPR3 expression within oocytes. Injection of small interfering double-stranded RNA (siRNA) targeting GPR3 stimulated meiotic resumption of oocytes. On the other hand, overexpression of GPR3 inhibited meiotic maturation of porcine oocytes, which was caused by increase of cGMP and cAMP levels and inhibition of cyclin B accumulation. Furthermore, incubation of porcine oocytes with the GPR3 ligand sphingosylphosphorylcholine (SPC) inhibited oocyte maturation. We propose that GPR3 is required for maintenance of meiotic arrest in porcine oocytes through pathways involved in the regulation of cAMP and cGMP.

Novel signaling mechanisms in the ovary during oocyte maturation and ovulation

During the peri-ovulatory period, the gonadotropin LH triggers major changes in both the somatic and germ cell compartments of the ovarian follicle. The oocyte completes the meiotic cell cycle to become a fertilizable egg, and dramatic changes in gene expression and secretion take place in the somatic compartment of the follicle in preparation for follicular rupture and oocyte release. The concerted changes are regulated by activation of intracellular signaling pathways as well as paracrine and autocrine regulatory loops. This review will provide a summary of the current knowledge of the molecular events triggered by LH focusing mostly on the signaling pathways required for oocyte maturation.

Hormonal control of mammalian oocyte meiosis at diplotene stage

Mammalian oocytes grow and undergo meiosis within ovarian follicles. Fully grown oocytes are arrested at the first meiotic prophase by a mural granulosa origin "arrester" until a surge of luteinizing hormone (LH) from the pituitary at the mid-cycle stimulates the immature oocyte to resume meiosis. Recent evidence indicates that natriuretic peptide precursor type C (NPPC) produced by mural granulosa cells stimulates the generation of cyclic guanosine 3',5'-monophosphate (cGMP) by cumulus cell natriuretic peptide receptor 2 (NPR2), which diffuses into oocyte via gap junctions and inhibits oocyte phosphodiesterase 3A (PDE3A) activity and cyclic adenosine 3',5'-monophosphate (cAMP) hydrolysis and maintains meiotic arrest with a high intraoocyte cAMP level. This cAMP is generated through the activity of the Gs G-protein by the G-protein-coupled receptor, GPR3 and GPR12, and adenylyl cyclases (ADCY) endogenous to the oocyte. Further studies suggest that endocrine hormones, such as follicle-stimulating hormone (FSH), LH, 17β-estradiol (E2) and oocyte-derived paracrine factors (ODPFs), participate in oocyte meiosis possibly by the regulation of NPPC and/or NPR2. A detailed investigation of NPPC and NPR2 expression in follicle cells will elucidate the precise molecular mechanisms of gonadotropins, and control the arrest as well as resumption of meiosis.

Developmental control of oocyte maturation and egg activation in metazoan models

Production of functional eggs requires meiosis to be coordinated with developmental signals. Oocytes arrest in prophase I to permit oocyte differentiation, and in most animals, a second meiotic arrest links completion of meiosis to fertilization. Comparison of oocyte maturation and egg activation between mammals, Caenorhabditis elegans, and Drosophila reveal conserved signaling pathways and regulatory mechanisms as well as unique adaptations for reproductive strategies. Recent studies in mammals and C. elegans show the role of signaling between surrounding somatic cells and the oocyte in maintaining the prophase I arrest and controlling maturation. Proteins that regulate levels of active Cdk1/cyclin B during prophase I arrest have been identified in Drosophila. Protein kinases play crucial roles in the transition from meiosis in the oocyte to mitotic embryonic divisions in C. elegans and Drosophila. Here we will contrast the regulation of key meiotic events in oocytes.

Anaphase-promoting complex control in female mouse meiosis

Entry into, and passage through, the two meiotic divisions of the oocyte has to be highly coordinated to ensure proper segregation of chromosomes. This coordination ensures that the hallmark stops and starts of the meiotic process occur at the right time to prevent aneuploidy. The Anaphase-Promoting Complex is an activity mostly studied in the mitotic cell cycle division, where it has essential functions during mitosis. As detailed here the Anaphase-Promoting Complex also plays vital roles in controlling at least three meiotic events: maintenance of prophase I arrest, timely and faithful segregation of homologous chromosomes in meiosis I, and the meiotic arrest following ovulation.

Genetic dissection of epidermal growth factor receptor signaling during luteinizing hormone-induced oocyte maturation

Recent evidence that luteinizing hormone (LH) stimulation of ovulatory follicles causes transactivation of the epidermal growth factor receptor (EGFR) has provided insights into the mechanisms of ovulation. However, the complete array of signals that promote oocyte reentry into the meiotic cell cycle in the follicle are still incompletely understood. To elucidate the signaling downstream of EGFR involved in oocyte maturation, we have investigated the LH responses in granulosa cells with targeted ablation of EGFR. Oocyte maturation and ovulation is disrupted when EGFR expression is progressively reduced. In granulosa cells from mice with either global or granulosa cell-specific disruption of EGFR signaling, LH-induced phosphorylation of MAPK3/1, p38MAPK, and connexin-43 is impaired. Although the LH-induced decrease in cGMP is EGFR-dependent in wild type follicles, LH still induces a decrease in cGMP in Egfr^delta/f Cyp19-Cre follicles. Thus compensatory mechanisms appear activated in the mutant. Spatial propagation of the LH signal in the follicle also is dependent on the EGF network, and likely is important for the control of signaling to the oocyte. Thus, multiple signals and redundant pathways contribute to regulating oocyte reentry into the cell cycle.

Endocytosis in the mouse oocyte and its contribution to cAMP signaling during meiotic arrest

Mammalian oocytes are arrested at prophase I of meiosis until a preovulatory surge of LH stimulates them to resume meiosis. Prior to the LH surge, high levels of cAMP within the oocyte maintain meiotic arrest; this cAMP is generated in the oocyte through the activity of the constitutively active, G(s)-coupled receptor, G-protein-coupled receptor 3 (GPR3) or GPR12. Activated GPRs are typically targeted for desensitization through receptor-mediated endocytosis, but a continuously high level of cAMP is needed for meiotic arrest. The aim of this study was to examine whether receptor-mediated endocytosis occurs in the mouse oocyte and whether this could affect the maintenance of meiotic arrest. We found that constitutive endocytosis occurs in the mouse oocyte. Inhibitors of receptor-mediated endocytosis, monodansylcadaverine and dynasore, inhibited the formation of early endosomes and completely inhibited spontaneous meiotic resumption. A red fluorescent protein-tagged GPR3 localized in the plasma membrane and within early endosomes in the oocyte, demonstrating that GPR3 is endocytosed. However, overexpression of G-protein receptor kinase 2 and β-arrestin-2 had only a modest effect on stimulating meiotic resumption, suggesting that these proteins do not play a major role in GPR3 endocytosis. Inhibition of endocytosis elevated cAMP levels within oocytes, suggesting that there is an accumulation of GPR3 at the plasma membrane. These results show that endocytosis occurs in the oocyte, leading to a decrease in cAMP production, and suggest that there is a balance between cAMP production and degradation in the arrested oocyte that maintains cAMP levels at an appropriate level during the maintenance of meiotic arrest.

The APC/C activator FZR1 coordinates the timing of meiotic resumption during prophase I arrest in mammalian oocytes

FZR1, an activator of the anaphase-promoting complex/cyclosome (APC/C), is recognized for its roles in the mitotic cell cycle. To examine its meiotic function in females we generated an oocyte-specific knockout of the Fzr1 gene (Fzr1Δ/Δ). The total number of fully grown oocytes enclosed in cumulus complexes was 35-40% lower in oocytes from Fzr1Δ/Δ mice and there was a commensurate rise in denuded, meiotically advanced and/or fragmented oocytes. The ability of Fzr1Δ/Δ oocytes to remain prophase I/germinal vesicle (GV) arrested in vitro was also compromised, despite the addition of the phosphodiesterase milrinone. Meiotic competency of smaller diameter oocytes was also accelerated by Fzr1 loss. Cyclin B1 levels were elevated ~5-fold in Fzr1Δ/Δ oocytes, whereas securin and CDC25B, two other APC/CFZR1 substrates, were unchanged. Cyclin B1 overexpression can mimic the effects of Fzr1 loss on GV arrest and here we show that cyclin B1 knockdown in Fzr1Δ/Δ oocytes affects the timing of meiotic resumption. Therefore, the effects of Fzr1 loss are mediated, at least in part, by raised cyclin B1. Thus, APC/CFZR1 activity is required to repress cyclin B1 levels in oocytes during prophase I arrest in the ovary, thereby maintaining meiotic quiescence until hormonal cues trigger resumption.

Regulation of the G2/M transition in rodent oocytes

Regulation of maturation in meiotically competent mammalian oocytes is a complex process involving the carefully coordinated exchange of signals between the somatic and germ cell compartments of the ovarian follicle via paracrine and cell-cell coupling pathways. This review highlights recent advances in our understanding of how such signaling controls both meiotic arrest and gonadotropin-triggered meiotic resumption in competent oocytes and relates them to the historical context. Emphasis will be on rodent systems, where many of these new findings have taken place. A regulatory scheme is then proposed that integrates this information into an overall framework for meiotic regulation that demonstrates the complex interplay between different follicular compartments.

Deletion of the novel oocyte-enriched gene, Gpr149, leads to increased fertility in mice

Through in silico subtraction and microarray analysis, we identified mouse Gpr149, a novel, oocyte-enriched transcript that encodes a predicted orphan G-protein-coupled receptor (GPR). Phylogenetic analysis of GPR149 from fish to mammals suggests that it is widely conserved in vertebrates. By multitissue RT-PCR analysis, we found that Gpr149 is highly expressed in the ovary and also in the brain and the digestive tract at low levels. Gpr149 levels are low in newborn ovaries but increase throughout folliculogenesis. In the ovary, we found that granulosa cells did not express Gpr149, whereas germinal vesicle and meiosis II stage oocytes showed high levels of Gpr149 expression. After fertilization, Gpr149 expression declined, becoming undetectable by the two-cell stage. To study the function of GPR149 in oocyte growth and maturation, we generated Gpr149 null mice. Surprisingly, Gpr149 null mice are viable and have normal folliculogenesis, but demonstrate increased fertility, enhanced ovulation, increased oocyte Gdf9 mRNA levels, and increased levels of FSH receptor and cyclin D2 mRNA levels in granulosa cells. Thus, Gpr149 null mice are one of the few models with enhanced fertility, and GPR149 could be a target for small molecules to enhance fertility in the assisted reproductive technology clinic.

cAMP pulsing of denuded mouse oocytes increases meiotic resumption via activation of AMP-activated protein kinase

cAMP plays a critical role in the control of oocyte maturation, as a high level of cAMP maintains oocyte arrest at the first meiotic prophase. Yet this study shows that pulsing meiotically arrested denuded oocytes (DO) with cAMP induces oocyte maturation through the activation of AMP-activated protein kinase (PRKA). Short-term (3 h) pulsing of meiotically arrested oocytes with forskolin, an adenyl cyclase (AC) activator, increased oocyte cAMP, led to elevated AMP, and induced oocyte meiotic resumption compared to oocytes continuously cultured in the control medium with or without forskolin. Western analysis showed that germinal vesicle (GV)-stage oocytes after forskolin pulsing contained increased levels of phospho-acetyl CoA carboxylase (pACACA), a primary substrate of PRKA. Pulsing oocytes with the phosphodiesterase (PDE)-sensitive cAMP analog, 8-bromo-cAMP (8-Br-cAMP), also increased pACACA and pPRKA levels in GV-stage oocytes and induced oocyte meiotic resumption. Moreover, the PRKA inhibitors, compound C and araA, prevented 8-Br-cAMP pulsing-induced maturation. The lack of effect on meiotic induction and PRKA activation when oocytes were pulsed with the PDE-resistant activators of cAMP-dependent protein kinase, Sp-cAMP-AM and Sp-5,6-DCI-cBIMPS, suggests that cAMP degradation is required for pulsing-induced maturation. Pulsing oocytes with the exchange protein directly activated by cAMP (Epac)-specific activator, 8-CPT-2'-O-Me-cAMP, had no stimulatory effect on oocyte maturation, suggesting Epac is not involved in the pulsing-induced maturation. Taken together, these data support the idea that a transient increase in oocyte cAMP can induce meiotic resumption via activation of PRKA.

The mammalian ovary from genesis to revelation

Two major functions of the mammalian ovary are the production of germ cells (oocytes), which allow continuation of the species, and the generation of bioactive molecules, primarily steroids (mainly estrogens and progestins) and peptide growth factors, which are critical for ovarian function, regulation of the hypothalamic-pituitary-ovarian axis, and development of secondary sex characteristics. The female germline is created during embryogenesis when the precursors of primordial germ cells differentiate from somatic lineages of the embryo and take a unique route to reach the urogenital ridge. This undifferentiated gonad will differentiate along a female pathway, and the newly formed oocytes will proliferate and subsequently enter meiosis. At this point, the oocyte has two alternative fates: die, a common destiny of millions of oocytes, or be fertilized, a fate of at most approximately 100 oocytes, depending on the species. At every step from germline development and ovary formation to oogenesis and ovarian development and differentiation, there are coordinated interactions of hundreds of proteins and small RNAs. These studies have helped reproductive biologists to understand not only the normal functioning of the ovary but also the pathophysiology and genetics of diseases such as infertility and ovarian cancer. Over the last two decades, parallel progress has been made in the assisted reproductive technology clinic including better hormonal preparations, prenatal genetic testing, and optimal oocyte and embryo analysis and cryopreservation. Clearly, we have learned much about the mammalian ovary and manipulating its most important cargo, the oocyte, since the birth of Louise Brown over 30 yr ago.