Regulatory contribution of heterotrimeric G-proteins to oocyte maturation in the sea urchin

In vitro induction of catfish, Clarias batrachus, oocyte maturation by conspecific vitellogenin 1 (CFVg1)

Present study demonstrates that conspecific vitellogenin1 (CFVg1) induces oocyte maturation in the catfish, Clarias batrachus. CFVg1 is able to develop fertilizable eggs in the Clarias batrachus. Therefore, different in vitro oocyte culture experiments were designed to see whether CFVg1 has efficacy of oocyte maturation and its pathway. In in vitro oocyte culture experiment, CFVg1 showed a dose- and time-dependent response and 64% maturation was obtained at the dose level of 10 µg/ml or more. CFVg1 induction of oocyte maturation was confirmed by co-incubating CFVg1 with CFVg1-antiserum (a-CFVg1), which inhibited the CFVg1-induced oocyte maturation. To answer issues lead to the understanding of the mechanism of vitellogenin (Vg) on oocyte maturation, trypsin digested CFVg1 and Indian major carp Cirhinus mrigala Vg HAI (Hydroxy appetite peak I) also showed significant level of maturation. Actinomycin-D and cycloheximide blocked the effect of CFVg1, indicating that CFVg1 acts through transcription and translation. Theophylline, the phosphodiesterase inhibitor, and cAMP also inhibited the stimulatory effect of CFVg1 on oocyte maturation, indicating indirectly that CFVg1-induced oocyte maturation by decreasing the intracellular cAMP possibly by activating the phosphodiesterase enzyme. Trilostane, the 3β-HSD-blocker, did not inhibit the CFVg1-induced oocyte maturation but wortmannin and Ly294002 two mechanistically different specific inhibitors of PI3 kinase blocked the oocyte maturation. The results thus indicate that oocyte maturation in catfish by Vg may be regulated by two pathways: (1) through decreasing the intraoocyte cAMP level by activating the cAMP-PKA pathway and (2) by cAMP-dependent PI3K/Akt pathway. Therefore, there might be role of vitellogenin itself in initiation of oocyte maturation.

Micromere formation and its evolutionary implications in the sea urchin

The micromeres of the sea urchin embryo are distinct from other blastomeres. After they arise through an asymmetric cell division at the 8- to 16-cell stage, micromeres immediately function as organizers. They also commit themselves to specific cell fates such as larval skeletogenic cells and primordial germ cells, while other blastomeres remain plastic and uncommitted at the 16-cell stage. In the phylum Echinodermata, only the sea urchin (class Echinoidea) embryo forms micromeres that serve as apparent organizers during early embryogenesis. Therefore, it is considered that micromeres are the derived features and that modification(s) of the developmental system allowed evolutionary introduction of this unique cell lineage. In this chapter, we summarize the both historic and recent observations that demonstrate unique properties of micromeres and discuss how this lineage of micromeres may have arisen during echinoderm evolution.

Evolutionary modification of AGS protein contributes to formation of micromeres in sea urchins

Evolution is proposed to result, in part, from acquisition of new developmental programs. One such example is the appearance of the micromeres in a sea urchin that form by an asymmetric cell division at the 4th embryonic cleavage and function as a major signaling center in the embryo. Micromeres are not present in other echinoderms and thus are  considered as a derived feature, yet its acquisition mechanism is unknown. Here, we report that the polarity factor AGS and its associated proteins are responsible for micromere formation. Evolutionary modifications of AGS protein seem to have provided the cortical recruitment and binding of AGS to the vegetal cortex, contributing to formation of micromeres in the sea urchins. Indeed, introduction of sea urchin AGS into the sea star embryo induces asymmetric cell divisions, suggesting that the molecular evolution of AGS protein is key in the transition of echinoderms to micromere formation and the current developmental style of sea urchins not seen in other echinoderms.

De novo transcriptome sequencing and comparative analysis to discover genes involved in ovarian maturity in Strongylocentrotus nudus

Strongylocentrotus nudus is an edible sea urchin, mainly harvested in China. Correlation studies indicated that S. nudus with larger diameter have a prolonged marketing time and better palatability owing to their precocious gonads and extended maturation process. However, the molecular mechanism underlying this phenomenon is still unknown. Here, transcriptome sequencing was applied to study the ovaries of adult S. nudus with different shell diameters to explore the possible mechanism. In this study, four independent cDNA libraries were constructed, including two from the big size urchins and two from the small ones using a HiSeq™2500 platform. A total of 88,581 unigenes were acquired with a mean length of 1354bp, of which 66,331 (74.88%) unigenes could be annotated using six major publicly available databases. Comparative analysis revealed that 353 unigenes were differentially expressed (with log2(ratio)≥1, FDR≤0.001) between the two groups. Of these, 20 differentially expressed genes (DEGs) were selected to confirm the accuracy of RNA-seq data by quantitative real-time RT-PCR. Furthermore, gene ontology and KEGG pathway enrichment analyses were performed to find the putative genes and pathways related to ovarian maturity. Eight unigenes were identified as significant DEGs involved in reproduction related pathways; these included Mos, Cdc20, Rec8, YP30, cytochrome P450 2U1, ovoperoxidase, proteoliaisin, and rendezvin. Our research fills the gap in the studies on the S. nudus ovaries using transcriptome analysis.

Steroid-induced oocyte maturation in Indian shad Tenualosa ilisha (Hamilton, 1822) is dependent on phosphatidylinositol 3 kinase but not MAP kinase activation

Fully grown fish and amphibian oocytes exposed to a maturation-inducing steroid (MIS) activates multiple signal transduction pathways, leading to formation and activation of maturation-promoting factor (MPF) and induction of germinal vesicle breakdown (GVBD). The present study was to investigate if phosphatidylinositol 3 kinase (PI3 kinase) and mitogen-activated protein kinase (MAP kinase) activation are required for naturally occurring MIS, 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P)-induced cdc2 activation and oocyte maturation (OM) in Tenualosa ilisha. We observed that 17,20β-P-induced OM was significantly inhibited by PI3 kinase inhibitors Wortmannin and LY29400. 17,20 β-P was shown to activate PI3 kinase maximally at 90 min and cdc2 kinase at 16 h of treatment. Relative involvement of PI3 kinase, MAP kinase and cdc2 kinase in 17,20β-P-induced OM was examined. MAP kinase was rapidly phosphorylated and activated (60-120 min) after MIS treatment and this response preceded the activation of cdc2 kinase by several hours. A selective inhibitor of MAP kinase (MEK), PD98059, sufficiently blocked the phosphorylation and activation of MAP kinase. Inhibition of MAP kinase activity using PD98059 however, had no effect on MIS-induced cdc2 kinase activation and GVBD. These results demonstrate that activation of the PI3 kinase is required for 17,20β-P-induced cdc2 kinase activation and OM in T. ilisha. MAP kinase although was activated in response to 17,20β-P and PI3 kinase activation, it is not necessary for cdc2 activation and OM in this species.

Involvement of G protein and purines in Rhinella arenarum oocyte maturation

We investigated the participation of G(αi) protein and of intracellular cAMP levels on spontaneous and progesterone-mediated maturation in Rhinella arenarum fully grown follicles and denuded oocytes. Although progesterone is the established maturation inducer in amphibians, Rhinella arenarum oocytes obtained during the reproductive period (competent oocytes) resume meiosis with no need for an exogenous hormonal stimulus if deprived of their enveloping follicular cells, a phenomenon called spontaneous maturation. In amphibian oocytes, numerous signalling mechanisms have been involved in the rapid, non-genomic, membrane effects of progesterone, but most of these are not fully understood. The data presented here demonstrate that activation of the G(αi) protein by Mas-7 induced maturation in non-competent oocytes and also an increase in GVBD (germinal vesicle breakdown) in competent oocytes. Similar results were obtained with intact follicles independent of the season. The activation of adenylyl cyclase (AC) by forskolin seems to inhibit both spontaneous and progesterone-induced GVBD. In addition, the high intracellular levels of cAMP caused by activation of AC by forskolin treatment or addition of db-cAMP inhibited maturation that had been induced by Mas-7 and in a dose-dependent manner. Treatment with H-89, a protein kinase A (PKA) inhibitor, was able to trigger GVBD in a dose-dependent manner in non-competent oocytes and increased the percentages of GVBD in oocytes competent to mature spontaneously. The results obtained with whole follicles and denuded oocytes were similar, which suggested that effects on AC and PKA were not mediated by follicle cells. The fact that Mas-7 was able to induce maturation in non-competent oocytes in a similar manner to progesterone and to increase spontaneous maturation suggests that G(αi) activation could be an important step in meiosis resumption. Thus, the decrease in cAMP as a result of the regulation of the G proteins on AC and the inactivation of PKA by H-89 could contribute to the activation of MPF (maturation promoting factor) and induce maturation of the oocytes of Rhinella arenarum.

Involvement of GABAA receptor in Bufo arenarum oocyte maturation

Amphibian oocytes meiotic arrest is released under the stimulus of progesterone; this hormone interacts with the oocyte surface and starts a cascade of events leading to the activation of a cytoplasmic maturation promoting factor (MPF) that induces germinal vesicle breakdown (GVBD), chromosome condensation and extrusion of the first polar body. The aim of this work was to determine whether the activation of a GABAA receptor is able to induce GVBD in fully grown denuded oocytes of Bufo arenarum and to analyse its possible participation in progesterone-induced maturation. We also evaluated the role of purines and phospholipids in the maturation process induced by a GABAA receptor agonist such as muscimol. Our results indicated that the activation of the GABAA receptor by muscimol induces maturation in a dose- and time-dependent manner and that this activation is a genuine maturation that enables oocytes to form pronuclei. Assays with a receptor antagonist, picrotoxine, showed that the maturation induced by muscimol was inhibited. Treatment with picrotoxine, however, shows that the participation of GABAA receptor in progesterone-induced maturation is not significant. In addition, our results indicate that high intracellular levels of purines obtained by the use of db-AMPc and theophylline or the inhibition of the phosphatidylinositol 4,5-bisphosphate (PIP2 hydrolysis by neomycin and PIP2 turn over by LiCl, respectively, inhibited the maturation induced by muscimol. Treatment with H-7 indicated, however, that PKC activation is not necessary for GVBD induced by the GABAA receptor agonist. Results suggest that the transduction pathway used by the GABAA receptor to induce maturation is different from those used by progesterone.

Activator of G-protein signaling in asymmetric cell divisions of the sea urchin embryo

An asymmetric fourth cell division in the sea urchin embryo results in formation of daughter cells, macromeres and micromeres, with distinct sizes and fates. Several lines of functional evidence presented here, including pharmacological interference and dominant negative protein expression, indicate that heterotrimeric G protein Gi and its interaction partner, activator of G-protein signaling (AGS), are necessary for this asymmetric cell division. Inhibition of Gi signaling by pertussis toxin interferes with micromere formation and leads to defects in embryogenesis. AGS was isolated in a yeast two-hybrid screen with G alpha i as bait and was expressed in embryos localized to the cell cortex at the time of asymmetric divisions. Introduction of exogenous dominant-negative AGS protein, containing only G-protein regulatory (GPR) domains, selectively prevented the asymmetric division in normal micromere formation. These results support the growing evidence that AGS is a universal regulator of asymmetric cell divisions in embryos.

A functional genomic and proteomic perspective of sea urchin calcium signaling and egg activation

The sea urchin egg has a rich history of contributions to our understanding of fundamental questions of egg activation at fertilization. Within seconds of sperm-egg interaction, calcium is released from the egg endoplasmic reticulum, launching the zygote into the mitotic cell cycle and the developmental program. The sequence of the Strongylocentrotus purpuratus genome offers unique opportunities to apply functional genomic and proteomic approaches to investigate the repertoire and regulation of Ca(2+) signaling and homeostasis modules present in the egg and zygote. The sea urchin "calcium toolkit" as predicted by the genome is described. Emphasis is on the Ca(2+) signaling modules operating during egg activation, but the Ca(2+) signaling repertoire has ramifications for later developmental events and adult physiology as well. Presented here are the mechanisms that control the initial release of Ca(2+) at fertilization and additional signaling components predicted by the genome and found to be expressed and operating in eggs at fertilization. The initial release of Ca(2+) serves to coordinate egg activation, which is largely a phenomenon of post-translational modifications, especially dynamic protein phosphorylation. Functional proteomics can now be used to identify the phosphoproteome in general and specific kinase targets in particular. This approach is described along with findings to date. Key outstanding questions regarding the activation of the developmental program are framed in the context of what has been learned from the genome and how this knowledge can be applied to functional studies.

Lineage-specific expansions provide genomic complexity among sea urchin GTPases

In every organism, GTP-binding proteins control many aspects of cell signaling. Here, we examine in silico several GTPase families from the Strongylocentrotus purpuratus genome: the monomeric Ras superfamily, the heterotrimeric G proteins, the dynamin superfamily, the SRP/SR family, and the "protein biosynthesis" translational GTPases. Identified were 174 GTPases, of which over 90% are expressed in the embryo as shown by tiling array and expressed sequence tag data. Phylogenomic comparisons restricted to Drosophila, Ciona, and humans (protostomes, urochordates, and vertebrates, respectively) revealed both common and unique elements in the expected composition of these families. Galpha and dynamin families contain vertebrate expansions, consistent with whole genome duplications, whereas SRP/SR and translational GTPases are highly conserved. Unexpectedly, Ras superfamily analyses revealed several large (5+) lineage-specific expansions in the sea urchin. For Rho, Rab, Arf, and Ras subfamilies, comparing total human gene numbers to the number of sea urchin genes with vertebrate orthologs suggests reduced genomic complexity in the sea urchin. However, gene duplications in the sea urchin increase overall numbers such that total sea urchin gene numbers approximate vertebrate gene numbers for each monomeric GTPase family. These findings suggest that lineage-specific expansions may be an important component of genomic evolution in signal transduction.

G proteins in development

The focus of developmental biologists has expanded from the analysis of gene expression to include the analysis of cell signalling. Heterotrimeric G proteins (G proteins) mediate signalling from a superfamily of heptahelical receptors (G-protein-coupled receptors) to a smaller number of effector units that include adenylyl cyclases, phospholipase C and various ion channels. The convergence of developmental biology with cell signalling has now revealed overlaps in which G proteins mediate complex pathways in embryonic development.

Steroid-induced oocyte maturation in Atlantic croaker (Micropogonias undulatus) is dependent on activation of the phosphatidylinositol 3-kinase/Akt signal transduction pathway

Exposure of fully grown fish and amphibian oocytes to a maturation-inducing steroid (MIS) activates numerous signal transduction pathways to initiate the final stage of oocyte maturation. These events culminate in the activation of maturation-promoting factor and germinal vesicle breakdown (GVBD). In most species, exposure to MIS causes a transient decrease in oocyte cAMP levels. Whether this reduction in oocyte cAMP concentration is sufficient to induce GVBD is unclear. The current study tested the hypothesis that activation of cAMP-independent signal transduction pathways by the naturally occurring MIS, 17,20beta,21-trihydroxy-4-pregnen-3-one (20beta-S), is necessary for GVBD in Atlantic croaker (Micropogonias undulatus) oocytes. Results indicate that although 20beta-S treatment of oocyte membranes significantly reduced cAMP production, incubation of follicles with the cell-permeable cAMP-dependent protein kinase (Prka) inhibitors Rp-cAMP or KT5720 did not promote GVBD in the absence of 20beta-S. Additionally, treatment of follicles with the phosphodiesterase (Pde) inhibitors Cilostamide (Pde3) or Rolipram (Pde4) significantly reduced GVBD, but they were not able to completely block it. In contrast, pharmacologic inhibition of the cAMP-independent phosphatidylinositol 3-kinase (Pik3)/Akt signal transduction pathway using the Pik3 inhibitors Wortmannin or LY294002, or the Akt inhibitor ML-9, blocked 20beta-S-induced GVBD. Finally, mitogen-activated protein kinase (Mapk1/3) activity increased after treatment with 20beta-S; however, inhibition of Mapk1/3 activity using PD98059 or U0126 had no effect on GVBD. These results demonstrate that activation of cAMP-independent signaling pathways, especially the Pik3/Akt pathway, is necessary for 20beta-S-induced GVBD in Atlantic croaker oocytes.

Cloning and characterization of a phospholipase C-beta isoform from the sea urchin Lytechinus pictus

Calcium is a ubiquitous intracellular signaling molecule controlling a wide array of cellular processes including fertilization and egg activation. The mechanism for triggering intracellular Ca(2+) release in sea urchin eggs during fertilization is the generation of inositol-1,4,5-trisphosphate by phospholipase C (PLC) hydrolysis of phosphatidylinositol-4,5-bisphosphate. Of the five PLC isoforms identified in mammals (beta, gamma, delta, epsilon and zeta), only PLCgamma and PLCdelta have been detected in echinoderms. Here, we provide direct evidence of the presence of a PLCbeta isoform, named suPLCbeta, within sea urchin eggs. The coding sequence was cloned from eggs of Lytechinus pictus and determined to have the greatest degree of homology and identity with the mammalian PLCbeta4. The presence of suPLCbeta within the egg was verified using a specifically generated antibody. The majority of the enzyme is localized in the non-soluble fraction, presumably the plasma membrane of the unfertilized egg. This distribution remains unchanged 1 min postfertilization. Unlike PLCbeta4, suPLCbeta is activated by G protein betagamma subunits, and this activity is Ca(2+)-dependent. In contrast to all known PLCbeta enzymes, suPLCbeta is not activated by Galphaq-GTPgammaS subunit suggesting other protein regulators may be present in sea urchin eggs.

βγ subunits of heterotrimeric G-proteins contribute to Ca2+ release at fertilization in the sea urchin

A cytoplasmic Ca2+ transient is required for egg activation at fertilization in all animals. The pathway leading to release of Ca2+ from the endoplasmic reticulum in echinoderms includes activation of a SRC homolog, followed by phospholipase Cγ activation, and formation of inositol trisphosphate. However, the upstream activators or modulators of this signaling pathway are not known. We recently identified four Gα subunits of heterotrimeric G-proteins present in the sea urchin egg, and here we find that activation of G-proteins of the Gαs and Gαq type, but not Gαi or Gα12 type, is required for normal Ca2+ dynamics at fertilization. The effects of these G-proteins are mediated by the Gβγ subunits, occur upstream of the cytoplasmic Ca2+ release, and influence both the amplitude of Ca2+ release and the duration of the lag phase. We propose integration of the G-protein input into the framework of signaling at sea urchin fertilization.