Role for endocytosis of a constitutively active GPCR (GPR185) in releasing vertebrate oocyte meiotic arrest

Unraveling the interplay between PKA inhibition and Cdk1 activation during oocyte meiotic maturation

Oocytes are arrested in prophase I. In vertebrates, meiotic resumption is triggered by hormonal stimulation that results in cAMP-dependent protein kinase (PKA) downregulation leading to Cdk1 activation. Yet the pathways connecting PKA to Cdk1 remain unclear. Here, we identify molecular events triggered by PKA downregulation occurring upstream of Cdk1 activation. We describe a two-step regulation controlling cyclin B1 and Mos accumulation, which depends on both translation and stabilization. Cyclin B1 accumulation is triggered by PKA inhibition upstream of Cdk1 activation, while its translation requires Cdk1 activity. Conversely, Mos translation initiates in response to the hormone, but the protein accumulates only downstream of Cdk1. Furthermore, two successive translation waves take place, the first controlled by PKA inhibition and the second by Cdk1 activation. Notably, Arpp19, an essential PKA effector, does not regulate the early PKA-dependent events. This study elucidates how PKA downregulation orchestrates multiple pathways that converge toward Cdk1 activation and induce the oocyte G2/M transition.

Progesterone induces meiosis through two obligate co-receptors with PLA2 activity

The steroid hormone progesterone (P4) regulates multiple aspects of reproductive and metabolic physiology. Classical P4 signaling operates through nuclear receptors that regulate transcription. In addition, P4 signals through membrane P4 receptors (mPRs) in a rapid nongenomic modality. Despite the established physiological importance of P4 nongenomic signaling, its detailed signal transduction remains elusive. Here, using Xenopus oocyte maturation as a well-established physiological readout of nongenomic P4 signaling, we identify the lipid hydrolase ABHD2 (α/β hydrolase domain-containing protein 2) as an essential mPRβ co-receptor to trigger meiosis. We show using functional assays coupled to unbiased and targeted cell-based lipidomics that ABHD2 possesses a phospholipase A2 (PLA2) activity that requires both P4 and mPRβ. This PLA2 activity bifurcates P4 signaling by inducing mPRβ clathrin-dependent endocytosis and producing lipid messengers that are G-protein coupled receptors agonists. Therefore, P4 drives meiosis by inducing the ABHD2 PLA2 activity that requires both mPRβ and ABHD2 as obligate co-receptors.

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.

Membrane progesterone receptor induces meiosis in Xenopus oocytes through endocytosis into signaling endosomes and interaction with APPL1 and Akt2

The steroid hormone progesterone (P4) mediates many physiological processes through either nuclear receptors that modulate gene expression or membrane P4 receptors (mPRs) that mediate nongenomic signaling. mPR signaling remains poorly understood. Here we show that the topology of mPRβ is similar to adiponectin receptors and opposite to that of G-protein-coupled receptors (GPCRs). Using Xenopus oocyte meiosis as a well-established physiological readout of nongenomic P4 signaling, we demonstrate that mPRβ signaling requires the adaptor protein APPL1 and the kinase Akt2. We further show that P4 induces clathrin-dependent endocytosis of mPRβ into signaling endosome, where mPR interacts transiently with APPL1 and Akt2 to induce meiosis. Our findings outline the early steps involved in mPR signaling and expand the spectrum of mPR signaling through the multitude of pathways involving APPL1.

The steroid hormone progesterone mediates many physiological processes through either nuclear receptors that modulate gene expression, or membrane progesterone receptors (mPRs) that mediate non-genomic signaling. This study shows that non-genomic mPRβ signaling progresses through clathrin-dependent endocytosis into signaling endosomes where it interacts with and activates APPL1 and Akt2 to induce oocyte meiosis.

Managing the Oocyte Meiotic Arrest-Lessons from Frogs and Jellyfish

During oocyte development, meiosis arrests in prophase of the first division for a remarkably prolonged period firstly during oocyte growth, and then when awaiting the appropriate hormonal signals for egg release. This prophase arrest is finally unlocked when locally produced maturation initiation hormones (MIHs) trigger entry into M-phase. Here, we assess the current knowledge of the successive cellular and molecular mechanisms responsible for keeping meiotic progression on hold. We focus on two model organisms, the amphibian Xenopus laevis, and the hydrozoan jellyfish Clytia hemisphaerica. Conserved mechanisms govern the initial meiotic programme of the oocyte prior to oocyte growth and also, much later, the onset of mitotic divisions, via activation of two key kinase systems: Cdk1-Cyclin B/Gwl (MPF) for M-phase activation and Mos-MAPkinase to orchestrate polar body formation and cytostatic (CSF) arrest. In contrast, maintenance of the prophase state of the fully-grown oocyte is assured by highly specific mechanisms, reflecting enormous variation between species in MIHs, MIH receptors and their immediate downstream signalling response. Convergence of multiple signalling pathway components to promote MPF activation in some oocytes, including Xenopus, is likely a heritage of the complex evolutionary history of spawning regulation, but also helps ensure a robust and reliable mechanism for gamete production.

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.

Role of Major Endocannabinoid-Binding Receptors during Mouse Oocyte Maturation

Endocannabinoids are key-players of female fertility and potential biomarkers of reproductive dysfunctions. Here, we investigated localization and expression of cannabinoid receptor type-1 and -2 (CB₁R and CB₂R), G-protein coupled receptor 55 (GPR55), and transient receptor potential vanilloid type 1 channel (TRPV1) in mouse oocytes collected at different stages of in vivo meiotic maturation (germinal vesicle, GV; metaphase I, MI; metaphase II, MII) through qPCR, confocal imaging, and western blot. Despite the significant decrease in CB₁R, CB₂R, and GPR55 mRNAs occurring from GV to MII, CB₂R and GPR55 protein contents increased during the same period. At GV, only CB₁R was localized in oolemma, but it completely disappeared at MI. TRPV1 was always undetectable. When oocytes were in vitro matured with CB₁R and CB₂R but not GPR55 antagonists, a significant delay of GV breakdown occurred, sustained by elevated intraoocyte cAMP concentration. Although CBRs antagonists did not affect polar body I emission or chromosome alignment, GPR55 antagonist impaired in ~75% of oocytes the formation of normal-sized MI and MII spindles. These findings open a new avenue to interrogate oocyte pathophysiology and offer potentially new targets for the therapy of reproductive alterations.

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.

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.

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.