Cytoplasmic and nuclear phospholipase C-beta 1 relocation: role in resumption of meiosis in the mouse oocyte

The role of myo-inositol supplement in assisted reproductive techniques

Assisted reproductive techniques can help many infertile couples conceive. Therefore, there is a need for an effective method to overcome the widespread problems of infertile men and women. Oocyte and sperm quality can increase the chances of successful in vitro fertilisation. The maturation environment in which gametes are present can affect their competency for fertilisation. It is well established that myo-inositol (MI) plays a pivotal role in reproductive physiology. It participates in cell membrane formation, lipid synthesis, cell proliferation, cardiac regulation, metabolic alterations, and fertility. This molecule also acts as a direct messenger of insulin and improves glucose uptake in various reproductive tissues. Evidence suggests that MI regulates events such as gamete maturation, fertilisation, and embryo growth through intracellular Ca2 + release and various signalling pathways. In addition to the in-vivo production of MI from glucose in the reproductive organs, its synthesis by in vitro-cultured sperm and follicles has also been reported. Therefore, MI is suggested as a therapeutic approach to maintain sperm and oocyte health in men and women with reproductive disorders and individuals of reproductive age.

Single cell RNA-seq reveals molecular pathways altered by 7, 12-dimethylbenz[a]anthracene treatment on pig oocytes

Oocytes of better quality and developmental competence are highly demanded, which is affected by many intrinsic and external factors, including environmental pollutants. We have previously demonstrated that 7, 12-dimethylbenz [a]anthracene (DMBA) reduces the developmental competence of porcine oocytes, by desynchronizing nuclear and ooplasmic maturation. However, the underlying molecular mechanism remains obscure. Here we performed single cell RNA-seq to study the transcriptome changes in DMBA-treated porcine MII oocytes, and identified 19 protein-coding genes and 156 novel long non-coding RNAs (lncRNAs) with abundance to be significantly different (P < 0.05), which enriched in signaling pathways such as glycosphingolipid biosynthesis, nicotine addiction, basal transcription factors and nucleotide excision repair. RT-qPCR on oocyte pools confirmed ornithine aminotransferase (Oat) and serine/arginine-rich splicing factor 4 (Srsf4) to be significantly up- and down-regulated, respectively (P < 0.05). Treating porcine COCs with MAPK and PLC pathway inhibitors suppressed DMBA's effects on increasing PB1 extrusion rate. In addition, DMBA co-incubation with 250 μM vitamin C derivative (l-ascorbic acid 2-phosphate sesquimagnesium salt hydrate, AA2P) and 100 μM co-enzyme Q10 (CoQ10) could significantly reduce the DMBA-induced high ROS level, and partially alleviate the DMBA-induced high PB1 rate, whereas the cleavage and blastocyst rates of parthenotes derived from treated mature oocytes remained to be low. Collectively, our findings indicate that single cell RNA-seq can help reveal the dynamics of molecular signaling pathways for porcine oocytes treated by DMBA, and supplement of anti-oxidative reagents could not sufficiently rescue DMBA-induced defects of porcine oocytes.

Constitutive IP3R1-mediated Ca2+ release reduces Ca2+ store content and stimulates mitochondrial metabolism in mouse GV oocytes

In mammals, fertilization initiates Ca²⁺ oscillations in metaphase II oocytes, which are required for the activation of embryo development. Germinal vesicle (GV) oocytes also display Ca²⁺ oscillations, although these unfold spontaneously in the absence of any known agonist(s) and their function remains unclear. We found that the main intracellular store of Ca²⁺ in GV oocytes, the endoplasmic reticulum ([Ca²⁺]_ER), constitutively 'leaks' Ca²⁺ through the type 1 inositol 1,4,5-trisphosphate receptor. The [Ca²⁺]_ER leak ceases around the resumption of meiosis, the GV breakdown (GVBD) stage, which coincides with the first noticeable accumulation of Ca²⁺ in the stores. It also concurs with downregulation of the Ca²⁺ influx and termination of the oscillations, which seemed underpinned by the inactivation of the putative plasma membrane Ca²⁺ channels. Lastly, we demonstrate that mitochondria take up Ca²⁺ during the Ca²⁺ oscillations, mounting their own oscillations that stimulate the mitochondrial redox state and increase the ATP levels of GV oocytes. These distinct features of Ca²⁺ homeostasis in GV oocytes are likely to underpin the acquisition of both maturation and developmental competence, as well as fulfill stage-specific cellular functions during oocyte maturation.

Transcriptomic features of Pecten maximus oocyte quality and maturation

The king scallop Pecten maximus is a high valuable species of great interest in Europe for both fishery and aquaculture. Notably, there has been an increased investment to produce seed for enhancement programmes of wild scallop populations. However, hatchery production is a relatively new industry and it is still underdeveloped. Major hurdles are spawning control and gamete quality. In the present study, a total of 14 scallops were sampled in the bay of Brest (Brittany, France) to compare transcriptomic profiles of mature oocytes collected by spawning induction or by stripping. To reach such a goal, a microarray analysis was performed by using a custom 8x60K oligonucleotide microarray representing 45,488 unique scallop contigs. First we identified genes that were differentially expressed depending on oocyte quality, estimated as the potential to produce D-larvae. Secondly, we investigated the transcriptional features of both stripped and spawned oocytes. Genes coding for proteins involved in cytoskeletal dynamics, serine/threonine kinases signalling pathway, mRNA processing, response to DNA damage, apoptosis and cell-cycle appeared to be of crucial importance for both oocyte maturation and developmental competence. This study allowed us to dramatically increase the knowledge about transcriptional features of oocyte quality and maturation, as well as to propose for the first time putative molecular markers to solve a major bottleneck in scallop aquaculture.

Comparative biology of sperm factors and fertilization-induced calcium signals across the animal kingdom

Fertilization causes mature oocytes or eggs to increase their concentrations of intracellular calcium ions (Ca²⁺) in all animals that have been examined, and such Ca²⁺ elevations, in turn, provide key activating signals that are required for non-parthenogenetic development. Several lines of evidence indicate that the Ca²⁺ transients produced during fertilization in mammals and other taxa are triggered by soluble factors that sperm deliver into oocytes after gamete fusion. Thus, for a broad-based analysis of Ca²⁺ dynamics during fertilization in animals, this article begins by summarizing data on soluble sperm factors in non-mammalian species, and subsequently reviews various topics related to a sperm-specific phospholipase C, called PLCζ, which is believed to be the predominant activator of mammalian oocytes. After characterizing initiation processes that involve sperm factors or alternative triggering mechanisms, the spatiotemporal patterns of Ca²⁺ signals in fertilized oocytes or eggs are compared in a taxon-by-taxon manner, and broadly classified as either a single major transient or a series of repetitive oscillations. Both solitary and oscillatory types of fertilization-induced Ca²⁺ signals are typically propagated as global waves that depend on Ca²⁺ release from the endoplasmic reticulum in response to increased concentrations of inositol 1,4,5-trisphosphate (IP₃). Thus, for taxa where relevant data are available, upstream pathways that elevate intraoocytic IP3 levels during fertilization are described, while other less-common modes of producing Ca²⁺ transients are also examined. In addition, the importance of fertilization-induced Ca²⁺ signals for activating development is underscored by noting some major downstream effects of these signals in various animals.

Distinct phospholipase C-β isozymes mediate lysophosphatidic acid receptor 1 effects on intestinal epithelial homeostasis and wound closure

Maintenance of the epithelial barrier in the intestinal tract is necessary to protect the host from the hostile luminal environment. Phospholipase C-β (PLC-β) has been implicated to control myriad signaling cascades. However, the biological effects of selective PLC-β isozymes are poorly understood. We describe novel findings that lysophosphatidic acid (LPA) regulates PLC-β1 and PLC-β2 via two distinct pathways to enhance intestinal epithelial cell (IEC) proliferation and migration that facilitate wound closure and recovery of the intestinal epithelial barrier. LPA acting on the LPA1 receptor promotes IEC migration by facilitating the interaction of Gαq with PLC-β2. LPA-induced cell proliferation is PLC-β1 dependent and involves translocation of Gαq to the nucleus, where it interacts with PLC-β1 to induce cell cycle progression. An in vivo study using LPA1-deficient mice (Lpar1(-/-)) shows a decreased number of proliferating IECs and migration along the crypt-luminal axis. Additionally, LPA enhances migration and proliferation of IECs in an LPA1-dependent manner, and Lpar1(-/-) mice display defective mucosal wound repair that requires cell proliferation and migration. These findings delineate novel LPA1-dependent lipid signaling that facilitates mucosal wound repair via spatial targeting of distinct PLC-βs within the cell.

Cellular neurochemical characterization and subcellular localization of phospholipase C β1 in rat brain

The present study describes a complete and detailed neuroanatomical distribution map of the phospholipase C beta1 (PLCβ1) isoform along the adult rat neuraxis, and defines the phenotype of cells expressing PLCβ1, along with its subcellular localization in cortical neurons as assessed by double-immunofluorescence staining and confocal laser scanning. Immunohistochemical labeling revealed a considerable morphological heterogeneity among PLCβ1-positive cells in the cortex, even though there was a marked predominance of pyramidal morphologies. As an exception to the general non-matching distribution of GFAP and PLCβ1, a high degree of co-expression was observed in radial glia-like processes of the spinal cord white matter. In the somatosensory cortex, the proportion of GABAergic neurons co-stained with PLCβ1 was similar (around 2/3) in layers I, II-III, IV and VI, and considerably lower in layer V (around 2/5). Double immunofluorescence against PLCβ1 and nuclear speckle markers SC-35 and NeuN/Fox3 in isolated nuclei from the rat cortex showed a high overlap of both markers with PLCβ1 within the nuclear matrix. In contrast, there was no apparent co-localization with markers of the nuclear envelope and lamina. Finally, to assess whether the subcellular expression pattern of PLCβ1 involved specifically one of the two splice variants of PLCβ1, we carried out Western blot experiments in cortical subcellular fractions. Notably, PLCβ1a/1b ratios were statistically higher in the cytoplasm than in the nuclear and plasma membrane fractions. These results provide a deeper knowledge of the cellular distribution of the PLCβ1 isoform in different cell subtypes of the rat brain, and of its presence in the neuronal nuclear compartment.

Nuclear phosphoinositides: location, regulation and function

Lipid signalling in human disease is an important field of investigation and stems from the fact that phosphoinositide signalling has been implicated in the control of nearly all the important cellular pathways including metabolism, cell cycle control, membrane trafficking, apoptosis and neuronal conduction. A distinct nuclear inositide signalling metabolism has been identified, thus defining a new role for inositides in the nucleus, which are now considered essential co-factors for several nuclear processes, including DNA repair, transcription regulation, and RNA dynamics. Deregulation of phoshoinositide metabolism within the nuclear compartment may contribute to disease progression in several disorders, such as chronic inflammation, cancer, metabolic, and degenerative syndromes. In order to utilize these very druggable pathways for human benefit there is a need to identify how nuclear inositides are regulated specifically within this compartment and what downstream nuclear effectors process and integrate inositide signalling cascades in order to specifically control nuclear function. Here we describe some of the facets of nuclear inositide metabolism with a focus on their relationship to cell cycle control and differentiation.

Expression analysis of regulatory microRNAs in bovine cumulus oocyte complex and preimplantation embryos

MicroRNAs (miRNAs) are small endogenous molecules that are involved in a diverse of cellular process. However, little is known about their abundance in bovine oocytes and their surrounding cumulus cells during oocyte development. To elucidate this situation, we investigated the relative expression pattern of sets of miRNAs between bovine oocyte and the surrounding cumulus cells during in vitro maturation using miRNA polymerase chain reaction (PCR) array. Results revealed that a total of 47 and 51 miRNAs were highly abundant in immature and matured oocytes, respectively, compared with their surrounding cumulus cells. Furthermore, expression analysis of six miRNAs enriched in oocyte miR-205, miR-150, miR-122, miR-96, miR-146a and miR-146b-5p at different maturation times showed a dramatic decrease in abundance from 0 h to 22 h of maturation. The expression of the same miRNAs in preimplantation stage embryos was found to be highly abundant in early stages of embryo development and decreased after the 8-cell stage to the blastocyst stage following a typical maternal transcript profile. Similar results were obtained by localization of miR-205 in preimplantation stage embryos, in which signals were higher up to the 4-cell stage and reduced thereafter. miR-205 and miR-210 were localized in situ in ovarian follicles and revealed a spatio-temporal expression during follicular development. Interestingly, the presence or absence of oocytes or cumulus cells during maturation was found to affect the expression of miRNAs in each of the two cell types. Hence, our results showed the presence of distinct sets of miRNAs in oocytes or cumulus cells and the presence of their dynamic degradation during bovine oocyte maturation.

Physiology and pathology of nuclear phospholipase C β1

The existence and function of inositide signaling in the nucleus is well documented and we know that the existence of the inositide cycle inside the nucleus has a biological role. An autonomous lipid-dependent signaling system, independently regulated from its plasma membrane counterpart, acts in the nucleus and modulates cell cycle progression and differentiation.We and others focused on PLCβ1, which is the most extensively investigated PLC isoform in the nuclear compartment. PLCβ1 is a key player in the regulation of nuclear inositol lipid signaling, and, as discussed above, its function could also be involved in nuclear structure because it hydrolyses PtdIns(4,5)P2, a well accepted regulator of chromatin remodelling. The evidence, in a number of patients with myelodysplastic syndromes, that the mono-allelic deletion of PLCβ1 is associated with an increased risk of developing acute myeloid leukemia paves the way for an entirely new field of investigation. Indeed the genetic defect evidenced, in addition to being a useful prognostic tool, also suggests that altered expression of this enzyme could have a role in the pathogenesis of this disease, by causing an imbalance between proliferation and apoptosis. The epigenetics of PLCβ1 expression in MDS has been reviewed as well.

Phospholipase C-beta1 signaling affects reproductive behavior, ovulation, and implantation

Infertility can result from a wide range of defects, from behavioral, through germ cell development and maturation, to fertilization or embryo development. Many of the hormones regulating these processes signal via G protein-coupled receptors, which in turn activate a range of plasma membrane enzymes including phospholipase C (PLC)-beta isoforms. Transgenic mice lacking functional Plc-beta1 (Plc-beta1 KO mice) have been noted to have severely impaired fertility, but there has been little study of the reproductive processes affected by lack of this enzyme. This study examined reproductive behavior, gonadal development, fertilization, and implantation in Plc-beta1 KO mice. Male and female Plc-beta1 KO mice exhibited impaired reproductive behavior. No other defect in reproduction was noted in males, raising the possibility that the reduced fertility of Plc-beta1 KO males could be due solely to impaired behavior. In contrast, female Plc-beta1 KO mice exhibited both behavioral and nonbehavioral defects. Plc-beta1 KO females ovulated only in response to exogenous hormones, with a large proportion of in vivo embryos recovered on embryonic d 4.5 exhibiting abnormal morphology. In addition, uteri of pregnant Plc-beta1 KO females exhibited an implantation defect, with poor embryo attachment and a failure to up-regulate cyclooxygenase-2 mRNA.

Involvement of nuclear PLCbeta1 in lamin B1 phosphorylation and G2/M cell cycle progression

Inositide-specific phospholipase Cbeta1 (PLCbeta1) signaling in cell proliferation has been investigated thoroughly in the G(1) cell cycle phase. However, little is known about its involvement in G(2)/M progression. We used murine erythroleukemia cells to investigate the role of PLCbeta1 in G(2)/M cell cycle progression and screened a number of candidate intermediate players, particularly mitogen-activated protein kinase (MAPK) and protein kinase C (PKC), which can, potentially, transduce serum mitogenic stimulus and induce lamin B1 phosphorylation, leading to G(2)/M progression. We report that PLCbeta1 colocalizes and physically interacts with lamin B1. Studies of the effects of inhibitors and selective si-RNA mediated silencing showed a role of JNK, PKCalpha, PKCbetaI, and the beta1 isoform of PI-PLC in cell accumulation in G(2)/M [as observed by fluorescence-activated cell sorter (FACS)]. To shed light on the mechanism, we considered that the final signaling target was lamin B1 phosphorylation. When JNK, PKCalpha, or PLCbeta1 were silenced, lamin B1 exhibited a lower extent of phosphorylation, as compared to control. The salient features to emerge from these studies are a common pathway in which JNK is likely to represent a link between mitogenic stimulus and activation of PLCbeta1, and, foremost, the finding that the PLCbeta1-mediated pathway represents a functional nuclear inositide signaling in the G(2)/M transition.

Alterations of PLCbeta1 in mouse eggs change calcium oscillatory behavior following fertilization

Inositol 1,4,5-trisphosphate generated by the action of a phospholipase C (PLC) mediates release of intracellular Ca2+ that is essential for sperm-induced activation of mammalian eggs. Much attention currently focuses on the role of sperm-derived PLCzeta in generating changes in egg intracellular Ca2+ despite the fact that PLCzeta constitutes a very small fraction of the total amount of PLC in a fertilized egg. Eggs express several isoforms of PLC, but a role for an egg-derived PLC in sperm-induced Ca2+ oscillations has not been examined. Reducing egg PLCbeta1 by a transgenic RNAi approach resulted in a significant decrease in Ca2+ transient amplitude, but not duration or frequency, following insemination. Furthermore, overexpressing PLCbeta1 by microinjecting a Plcb1 cRNA significantly perturbed the duration and frequency of Ca2+ transients and disrupted the characteristic shape of the first transient. These results provide the first evidence for a role of an egg-derived PLC acting in conjunction with a sperm-derived PLCzeta in egg activation.

The PKC pathway and in particular its β1 isoform is clearly involved in meiotic arrest maintenance but poorly in FSH-induced meiosis resumption of the mouse cumulus cell enclosed oocyte

PKC modulators were used to investigate the role of the PKC pathway either on the maintenance of meiotic arrest or on FSH-induced maturation of mouse cumulus cell enclosed oocytes (CEOs). (1) Whereas PKC activation (PMA 8 microM) overcomed clearly the HX-maintained meiotic arrest (83.7 +/- 3.6% vs. 16.1 +/- 10.6% GVBD oocytes), PKC inhibition (Calphostin C 100 nM) did not. On the contrary, it better maintained the meiotic arrest than HX alone. (2) No significant effect of PKC activation or inhibition was observed. (3) HX alone maintained PKCbeta1 in the cytoplasm, whereas FSH and PKC activation induced partly its translocation into the nucleus. The results show that whereas the PKC pathway is clearly involved in maintenance of the meiotic arrest through PKCbeta1, it is not involved in FSH-induced meiosis of CEOs.

Spontaneous calcium oscillations and nuclear PLC-β1 in human GV oocytes

Our aim was to investigate if human oocytes, like mouse oocytes, exhibit spontaneous Ca(2+) oscillations and nuclear translocation of PLC-beta1 prior to germinal vesicle breakdown (GVBD), and to correlate these events with the evolution of chromatin configuration as a landmark for the meiosis resumption kinetics. Human germinal vesicle (GV) oocytes were either loaded with Fluo-3 probe to record Ca(2+) signals or fixed for subsequent fluorescent labeling of both chromatin and PLC-beta1, and immunogold labeling of PLC-beta1. Here for the first time, we show that human oocytes at the GV-stage exhibit spontaneous Ca(2+) oscillations. Interestingly, only oocytes with a large diameter and characterized by a compact chromatin surrounding the nucleolus of the GV could reveal these kind of oscillations. We also observed a translocation of PLC-beta1 from the cytoplasm towards the nucleus during in vitro maturation of human oocytes. Spontaneous calcium oscillations and nuclear translocation of PLC-beta1 may reflect some degree of oocyte maturity. The impact of our results may be very helpful to understand and resolve many enigmatic problems usually encountered during the in vitro meiotic maturation of human GV oocytes.

The Role of X/Y Linker Region and N-terminal EF-hand Domain in Nuclear Translocation and Ca2+ Oscillation-inducing Activities of Phospholipase Cζ, a Mammalian Egg-activating Factor

Sperm-specific phospholipase C-zeta (PLCzeta) causes intracellular Ca(2+) oscillations and thereby egg activation and is accumulated into the formed pronucleus (PN) when expressed in mouse eggs by injection of cRNA encoding PLCzeta, which consists of four EF-hand domains (EF1-EF4) in the N terminus, X and Y catalytic domains, and C-terminal C2 domain. Those activities were analyzed by expressing PLCzeta mutants tagged with fluorescent protein Venus by injection of cRNA into unfertilized eggs or 1-cell embryos after fertilization. Nuclear localization signal (NLS) existed at 374-381 in the X/Y linker region. Nuclear translocation was lost by replacement of Arg(376), Lys(377), Arg(378), Lys(379), or Lys(381) with glutamate, whereas Ca(2+) oscillations were conserved. Nuclear targeting was also absent for point mutation of Lys(299) and/or Lys(301) in the C terminus of X domain, or Trp(13), Phe(14), or Val(18) in the N terminus of EF1. Ca(2+) oscillation-inducing activity was lost by the former mutation and was remarkably inhibited by the latter. A short sequence 374-383 fused with Venus showed active translocation into the nucleus of COS-7 cells, but 296-309 or 1-19 did not. Despite the presence of these special regions, both activities were deprived by deletion of not only EF1 but also EF2-4 or C2 domain. Thus, PLCzeta is driven into the nucleus primarily by the aid of NLS and putative regulatory sites, but coordinated three-dimensional structure, possibly formed by a folding in the X/Y linker and close EF/C2 contact as in PLCdelta1, seems to be required not only for enzymatic activity but also for nuclear translocation ability.

Calcium signals for egg activation in mammals

A dramatic increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) occurs in eggs at fertilization common to all animal species examined to date, and this serves as a pivotal signal for egg activation characterized by resumption of meiotic cell division and formation of the pronuclei. In mammalian eggs, repetitive [Ca(2+)](i) rises (Ca(2+) oscillations) each of which accompanies a propagating wave across the egg occur due to release of Ca(2+) from the endoplasmic reticulum mainly through type 1 inositol 1,4,5-trisphosphate (IP(3)) receptor. Ca(2+) oscillations are induced by a cytosolic sperm factor driven into the egg cytoplasm upon sperm-egg fusion. A current strong candidate of the sperm factor is a novel sperm-specific isozyme of phospholipase C (IP(3)-producing enzyme), PLCzeta. Recent extensive research has reveled characteristics of PLCzeta such as the Ca(2+) oscillation-inducing activity after injection of PLCzeta-encoding RNA or recombinant PLCzeta into mouse eggs, extremely high Ca(2+)-sensitivity of the enzymatic activity in vitro, and nuclear translocation ability possibly related to cell-cycle-dependent regulation of Ca(2+) oscillations. [Ca(2+)](i) rises cause successive activation of calmodulin-dependent kinase II and E3 ubiquitin ligase, lead to proteolysis of ubiquitinated cyclin B1 and inactivation of metaphase-promoting factor (Cdk1/cyclin B1 complex), and result in the release of eggs from meiotic arrest.

Lead cations affect the control of both meiosis arrest and meiosis resumption of the mouse oocyte in vitro at least via the PKC pathway

The aim of this study was to determine in vitro whether lead has a direct cytotoxic effect on the female gamete or through its surrounding somatic cells. We had previously demonstrated that it partly accumulates in the mouse ovary and induces follicle and oocyte apoptosis. The data reported here demonstrate for the first time that low levels of Pb(NO3)2 (<or=10 pM) affect oocyte meiosis in vitro. On the one hand, in condition similar to the in vivo one, i.e. in hypoxanthine (HX)-maintained meiotic arrest, Pb(NO3)2 was able to significantly release the oocytes from this arrest. On the other hand, when meiosis occurred spontaneously in vitro, Pb(NO3)2 inhibits meiosis. Whereas PMA, an agonist of PKC was able to prevent the first lead effect, calphostin C, an antagonist, was able to significantly prevent lead inhibition of spontaneous GVBD. And, similarly to Pb2+, the inhibition of PKC by calphostin C prevented HX to maintain the meiotic arrest whereas its activation by PMA inhibited spontaneous GVBD. No significant differences in the effects of Pb(NO3)2 on the oocytes were observed whatever the cumulus cells were present or absent. Moreover, lead did not seem to affect the metaphase plate formation. We concluded that Pb2+ may disturb the control of oocyte meiosis at least in part through its ability to interfere with the PKC pathway in taking place of Ca2+ ions.

Phosphoinositide-specific phospholipase C (PI-PLC) β1 and nuclear lipid-dependent signaling

Over the last years, evidence has suggested that phosphoinositides, which are involved in the regulation of a large variety of cellular processes both in the cytoplasm and in the plasma membrane, are present also within the nucleus. A number of advances has resulted in the discovery that phosphoinositide-specific phospholipase C signalling in the nucleus is involved in cell growth and differentiation. Remarkably, the nuclear inositide metabolism is regulated independently from that present elsewhere in the cell. Even though nuclear inositol lipids hydrolysis generates second messengers such as diacylglycerol and inositol 1,4,5-trisphosphate, it is becoming increasingly clear that in the nucleus polyphosphoinositides may act by themselves to influence pre-mRNA splicing and chromatin structure. Among phosphoinositide-specific phospholipase C, the beta(1) isoform appears to be one of the key players of the nuclear lipid signaling. This review aims at highlighting the most significant and up-dated findings about phosphoinositide-specific phospholipase C beta(1) in the nucleus.

Nuclear protein kinase C

Protein kinase C (PKC) isozymes constitute a family of ubiquitous phosphotransferases which act as key transducers in many agonist-induced signaling cascades. To date, at least 11 different PKC isotypes have been identified and are believed to play distinct regulatory roles. PKC isoforms are physiologically activated by a number of lipid cofactors. PKC is thought to reside in the cytoplasm in an inactive conformation and to translocate to the plasma membrane or cytoplasmic organelles upon cell activation by different stimuli. However, a sizable body of evidence collected over the last 20 years has shown PKC to be capable of translocating to the nucleus. Furthermore, PKC isoforms are resident within the nucleus. Studies from independent laboratories have to led to the identification of quite a few nuclear proteins which are PKC substrates and to the characterization of nuclear PKC-binding proteins which may be critical for finely tuning PKC function in this cell microenvironment. Several lines of evidence suggest that nuclear PKC isozymes are involved in the regulation of biological processes as important as cell proliferation and differentiation, gene expression, neoplastic transformation, and apoptosis. In this review, we shall highlight the most intriguing and updated findings about the functions of nuclear PKC isozymes.

Requirements of Src family kinase during meiotic maturation in mouse oocyte

The Src family kinase (SFK) is important in normal cell cycle control. However, its role in meiotic maturation in mammalian has not been examined. We used confocal microscope immunofluorescence to examine the in vitro dynamics of the subcellular distribution of SFK during the mouse oocyte meiotic maturation and further evaluated the functions of SFK via biochemical analysis using a specific SFK pharmacological inhibitor, PP(2). Our results showed that nonphospho-SFK was absent in oocyte upon its release from follicle. Nonphospho-SFK appeared in cytoplasm 0.5 hr after the release of oocyte and translocated to germinal vesicle (GV) before germinal vesicle breakdown (GVBD). After GVBD, nonphospho-SFK colocated with condensed chromosomes. In occyte at metaphase I (MI) and telophase I, nonphospho-SFK accumulated in the cortex and the cleavage furrow respectively besides its existence in cytoplasm in both stages. In oocyte at metaphase II (MII), nonphospho-SFK concentrated at the aligned chromosomes. In contrast, phospho-SFK was absent in oocyte until 1 hr after its release from the follicle. Phospho-SFK accumulated in the GV, the cortex, and cytoplasm immediately prior to GVBD. After GVBD, phospho-SFK evenly distributed in oocyte. In oocyte at MII, phospho-SFK localized throughout the cytoplasm and under the egg member. When the SFK activity was inhibited, the oocyte failed to initiate GVBD, could not go into MII, and could not extrude the first polar body. Our results demonstrated that SFK is required for meiotic maturation in mouse oocyte.

Nuclear translocation of phospholipase C-zeta, an egg-activating factor, during early embryonic development

Phospholipase C-zeta (PLCzeta), a strong candidate of the egg-activating sperm factor, causes intracellular Ca2+ oscillations and egg activation, and is subsequently accumulated into the pronucleus (PN), when expressed in mouse eggs by injection of RNA encoding PLCzeta. Changes in the localization of expressed PLCzeta were investigated by tagging with a fluorescent protein. PLCzeta began to translocate into the PN formed at 5-6 h after RNA injection and increased there. Observation in the same embryo revealed that PLCzeta in the PN dispersed to the cytoplasm upon nuclear envelope breakdown and translocated again into the nucleus after cleavage. The dynamics was found in the second mitosis as well. When RNA was injected into fertilization-originated 1-cell embryos or blastomere(s) of 2-8-cell embryos, the nuclear localization of expressed PLCzeta was recognized in every embryo up to blastocyst. Thus, PLCzeta exhibited alternative cytoplasm/nucleus localization during development. This supports the view that the sperm factor could control cell cycle-dependent generation of Ca2+ oscillations in early embryogenesis.

Phosphorylated MARCKS: A novel centrosome component that also defines a peripheral subdomain of the cortical actin cap in mouse eggs

MARCKS (myristoylated alanine-rich C-kinase substrate) is a major substrate for protein kinase C (PKC), a kinase that has multiple functions during oocyte maturation and egg activation, for example, spindle function and cytoskeleton reorganization. We examined temporal and spatial changes in p-MARCKS localization during maturation of mouse oocytes and found that p-MARCKS is a novel centrosome component based its co-localization with pericentrin and gamma-tubulin within microtubule organizing centers (MTOCs). Like pericentrin, p-MARCKS staining at the MI spindle poles was asymmetric. Based on this asymmetry, we found that one end of the spindle was preferentially extruded with the first polar body. At MII, however, the spindle poles had symmetrical p-MARCKS staining. p-MARCKS also was enriched in the periphery of the actin cap overlying the MI or MII spindle to form a ring-shaped subdomain. Because phosphorylation of MARCKS modulates its actin crosslinking function, this localization suggests p-MARCKS functions as part of the contractile apparatus during polar body emission. Our finding that an activator of conventional and novel PKC isoforms did not increase the amount of p-MARCKS suggested that an atypical isoform was responsible for MARCKS phosphorylation. Consistent with this idea, immunostaining revealed that the staining patterns of p-MARCKS and the active form of the atypical PKC zeta/lambda isoform(s) were very similar. These results show that p-MARCKS is a novel centrosome component and also defines a previously unrecognized subdomain of the actin cap overlying the spindle.

Disruption of the mouse phospholipase C-beta1 gene in a beta-lactoglobulin transgenic line affects viability, growth, and fertility in mice

A recessive insertional mutation was identified in one line of transgenic mice for the caprine beta-lactoglobulin (betaLG) gene. High mortality after birth, a significant reduction in postnatal growth and adult body size, changes in the morphometric features of the head, and infertility are the most prominent phenotypic traits of the mutant animals. Molecular cloning and sequencing of the transgene insertion site showed that 22 copies of the betaLG transgene are inserted in an intronic region of the phospholipase C-beta1 (PLC-beta1) gene, which plays a pivotal role in modulating different cellular functions. As a result of the insertional mutation (PLC-beta1(betaLG) mutation), a hybrid messenger RNA (mRNA) between the mouse PLC-beta1 and the goat betaLG genes is transcribed. The tissue-specific pattern of expression of this hybrid mRNA in PLC-beta1(betaLG) homozygotes is equivalent to that of the endogenous PLC-beta1 mRNA in nontransgenic animals, which is reported for the first time in this species, but expression levels are significantly reduced. Although the hybrid PLCbeta1-betaLG mRNA contains all the essential information to produce a PLCbeta1 protein that could be activated, this protein was not detected by Western blot. The PLC-beta1(betaLG) mouse model described here represents a useful tool to investigate the role of the PLC-beta1 gene in the molecular mechanisms underlying growth and fertility.

Neurotrophin Signaling in Oocyte Survival and Developmental Competence: A Paradigm for Cellular Toti-Potency

While not fulfilling the criterion of a "stem cell" in being capable of self-renewal, mature and fertilized oocytes are the original "toti-potent" cells, whose capacity for expansion and differentiation can only be approximated by stem cells of embryonic or adult origin in vitro. As such, the mechanisms by which oocytes acquire and manifest competence to support embryo development is of fundamental interest to efforts to control and re-specify somatic cell fate and toti-potency. This is underscored by the unparalleled capacity of oocyte cytoplasm to successfully re-specify the genetic program of animal development following cell nuclear replacement (i.e., cloning). Thus, the knowledge gained by understanding the acquisition of oocyte developmental competence could ultimately facilitate the creation of adult stem cells in vitro from terminally differentiated cells, ex ovo. In this paper, we review the concept of oocyte developmental competence, and focus on our own research and that of others implicating a role for neurotrophins in this process, and that of oocyte cell survival. Lastly we propose a role for neurotrophin signalling in embryo stem cell survival.

Regulation of spontaneous meiosis resumption in mouse oocytes by various conventional PKC isozymes depends on cellular compartmentalization

In this study, we investigated the spatio-temporal distribution of conventional protein kinases C (cPKC) isoforms PKC-alpha, PKC-betaI, PKC-betaII and PKC-gamma in mouse oocytes. The cPKCs were present in the cytoplasm at the start of the process and migrated to the nucleus (or germinal vesicle) before germinal vesicle breakdown, except for PKC-gamma which remained cytoplasmic. In both compartments, the fully phosphorylated form corresponding to the 'mature' enzyme was revealed for PKC-alpha, PKC-betaI and PKC-betaII. Microinjection of specific antibodies against each isozyme in one or the other cell compartment at different times of the meiotic process, permitted us to observe the following: (1) When located in the cytoplasm at the beginning of the process, PKC-alpha is not implicated in germinal vesicle breakdown, PKC-betaI and PKC-gamma are involved in maintaining the meiotic arrest, and PKC-betaII plays a role in meiosis reinitiation. Furthermore, just before germinal vesicle breakdown, these cytoplasmic cPKCs were no longer implicated. (2) When located in the germinal vesicle, PKC-alpha, PKC-betaI and PKC-betaII are involved in meiosis reinitiation. Our data highlight not only the importance of the nuclear pathways in the cell cycle progression, but also their independence of the cytoplasmic ones. Further investigations are however necessary to discover the molecular targets of these cPKCs to better understand the links with the cell cycle progression.

Ca2+ oscillation-inducing phospholipase C zeta expressed in mouse eggs is accumulated to the pronucleus during egg activation

Sperm-specific phospholipase C zeta (PLC zeta) is known to induce intracellular Ca(2+) oscillations and egg activation when expressed in mouse eggs by injection of RNA encoding PLC zeta. We investigated the expression level and spatial distribution of PLC zeta in the egg in real time and in relation to the initiation and termination of Ca(2+) oscillations by monitoring fluorescence of a yellow fluorescent protein 'Venus' fused with PLC zeta. Ca(2+) oscillations similar to those at fertilization were induced at 40-50 min after RNA injection, when expressed PLC zeta reached 10-40 x 10(-15) g in the egg. PLC zeta-Venus increased up to 3 h and attained a steady level at 4-5 h. Interestingly, PLC zeta-Venus is accumulated to the pronucleus (PN) formed at 5-6 h and continuously increased there. Ca(2+) oscillations stopped in most eggs before initiation of the accumulation. A variant of PLC zeta that lacks three EF hand domains was much less effective in induction of Ca(2+) oscillations and little accumulated in the pronucleus, indicating a critical role of those domains. The ability of the accumulation to the pronucleus qualifies PLC zeta for a strong candidate of the Ca(2+) oscillation-inducing sperm factor, which is introduced into the ooplasm upon sperm-egg fusion and concentrated to the pronucleus after inducing egg activation.

Nuclear inositides: facts and perspectives

Strong evidence has been accumulating over the last 15 years suggesting that phosphoinositides, which are involved in the regulation of a large variety of cellular processes in the cytoplasm and in the plasma membrane, are present within the nucleus. Several advances have resulted in the discovery that nuclear phosphoinositides are involved in cell growth and differentiation. Remarkably, the nuclear inositide metabolism is regulated independently from that present elsewhere in the cell. Although nuclear inositol lipids generate second messengers such as diacylglycerol and inositol 1,4,5-trisphosphate, it is becoming increasingly clear that in the nucleus polyphosphoinositides may act by themselves to influence pre-mRNA splicing and chromatin structure. This review aims at highlighting the most significant and updated findings about inositol lipid metabolism in the nucleus.

Nuclear phosphoinositides and their functions

Phosphoinositides are minor components of biological membranes, which have emerged as essential regulators of a variety of cellular processes, both on the plasma membrane and on several intracellular organelles. The versatility of these lipids stems from their ability to function either as substrates for the generation of second messengers, as membrane-anchoring sites for cytosolic proteins or as regulators of the actin cytoskeleton. Despite a vast literature demonstrating the presence of phosphoinositides in the nucleus, only recently has the function(s) of the nuclear pool of these lipids and their soluble analogues, inositol polyphosphates, started to emerge. These compounds have been shown to serve as essential co-factors for several nuclear processes, including DNA repair, transcription regulation and RNA dynamics. In this light, phosphoinositides and inositol polyphosphates might represent high turnover activity switches for nuclear complexes responsible for these processes. The regulation of these large machineries would be linked to the phosphorylation state of the inositol ring and limited temporally and spatially based on the synthesis and degradation of these molecules.

Meiosis resumption, calcium-sensitive period, and PLC-β1 relocation into the nucleus in the mouse oocyte

We aimed to determine whether the breakdown of the germinal vesicle of the mouse oocyte and the nuclear import of phospholipase C-beta1 were calcium-dependent. We chelated Ca2+ ions with BAPTA-dextran at different times after the release of the oocyte from the ovarian follicle, i.e. after meiosis resumption has started, and we studied the effects on the kinetics of germinal vesicle breakdown, and on the migration of phospholipase C-beta1. We discriminate between two key-periods of calcium-sensitivity during the process of meiosis resumption. During the first hour, changes in the cytosolic Ca2+ especially promoted the migration of phospholipase C-beta1 into the nucleus, whereas changes in the nuclear concentration of Ca2+ were not implicated. Moreover, at this time, the cytosolic calcium pathway is PLC-beta1-dependent. By contrast, during the second hour following the onset of meiosis resumption, and thus just previous GVBD, the PLC-beta1-dependent Ca2+ signals in both cellular compartments were equally necessary for the resumption of meiosis. This particular period of the meiotic process corresponds to the moment when the phospholipase C-beta1 has strongly migrated into the nucleus. Our results highlight also the role played by the nucleus during the second key-period in the control of the GVBD via a Ca2+-dependent pathway.

Nuclear lipid signalling

During the past twenty years, evidence has accumulated for the presence of phospholipids within the nuclei of eukaryotic cells. These phospholipids are distinct from those that are obviously present in the nuclear envelope. The best characterized of the intranuclear lipids are the inositol lipids that form the components of a phosphoinositide-phospholipase C cycle. However, exactly as has been discovered in the cytoplasm, this is just part of a complex picture that involves many other lipids and functions.

Ca2+ oscillations at fertilization in mammals are regulated by the formation of pronuclei

In mammals, the sperm triggers a series of cytosolic Ca(2+) oscillations that continue for approximately 4 hours, stopping close to the time of pronucleus formation. Ca(2+) transients are also seen in fertilized embryos during the first mitotic division. The mechanism that controls this pattern of sperm-induced Ca(2+) signalling is not known. Previous studies suggest two possible mechanisms: first, regulation of Ca(2+) oscillations by M-phase kinases; and second, regulation by the presence or absence of an intact nucleus. We describe experiments in mouse oocytes that differentiate between these mechanisms. We find that Ca(2+) oscillations continue after Cdk1-cyclin B1 activity falls at the time of polar body extrusion and after MAP kinase has been inhibited with UO126. This suggests that M-phase kinases are not necessary for continued Ca(2+) oscillations. A role for pronucleus formation in regulating Ca(2+) signalling is demonstrated in experiments where pronucleus formation is inhibited by microinjection of a lectin, WGA, without affecting the normal inactivation of the M-phase kinases. In oocytes with no pronuclei but with low M-phase kinase activity, sperm-induced Ca(2+) oscillations persist for nearly 10 hours. Furthermore, a dominant negative importin beta that inhibits nuclear transport, also prevents pronucleus formation and causes Ca(2+) oscillations that continue for nearly 12 hours. During mitosis, fluorescent tracers that mark nuclear envelope breakdown and the subsequent reformation of nuclei in the newly formed two-cell embryo establish that Ca(2+) oscillations are generated only in the absence of a patent nuclear membrane. We conclude by suggesting a model where nuclear sequestration and release of a Ca(2+)-releasing activity contributes to the temporal organization of Ca(2+) transients in meiosis and mitosis in mice.

Nuclear lipid signaling

Abundant evidence now supports the existence of phospholipids in the nucleus that resist washing of nuclei with detergents. These lipids are apparently not in the nuclear envelope as part of a bilayer membrane, but are actually within the nucleus in the form of proteolipid complexes with unidentified proteins. This review discusses the experimental evidence that attempts to explain their existence. Among these nuclear lipids are the polyphosphoinositol lipids which, together with the enzymes that synthesize them, form an intranuclear phospholipase C (PI-PLC) signaling system that generates diacylglycerol (DAG) and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. The isoforms of PI-PLC that are involved in this signaling system, and how they are regulated, are not yet entirely clear. Generation of DAG within the nucleus is believed to recruit protein kinase C (PKC) to the nucleus to phosphorylate intranuclear proteins. Generation of Ins(1,4,5)P3 may mobilize Ca2+ from the space between the nuclear membranes and thus increase nucleoplasmic Ca2+. Less well understood are the increasing number of variations and complications on the "simple" idea of a PI-PLC system. These include, all apparently within the nucleus, (i) two routes of synthesis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]; (ii) two sources of DAG, one from the PI-PLC pathway and the other probably from phosphatidylcholine; (iii) several isoforms of PKC translocating to nuclei; (iv) increases in activity of the PI-PLC pathway at two points in the cell cycle; (v) a pathway of phosphorylation of Ins(1,4,5)P3, which may have several functions, including a role in the transfer of mRNA out of the nucleus; and (vi) the possible existence of other lipid signaling pathways that may include sphingolipids, phospholipase A2, and, in particular, 3-phosphorylated inositol lipids, which are now emerging as possible major players in nuclear signaling.

Proliferating or differentiating stimuli act on different lipid-dependent signaling pathways in nuclei of human leukemia cells

Previous results have shown that the human promyelocytic leukemia HL-60 cell line responds to either proliferating or differentiating stimuli. When these cells are induced to proliferate, protein kinase C (PKC)-beta II migrates toward the nucleus, whereas when they are exposed to differentiating agents, there is a nuclear translocation of the alpha isoform of PKC. As a step toward the elucidation of the early intranuclear events that regulate the proliferation or the differentiation process, we show that in the HL-60 cells, a proliferating stimulus (i.e., insulin-like growth factor-I [IGF-I]) increased nuclear diacylglycerol (DAG) production derived from phosphatidylinositol (4,5) bisphosphate, as indicated by the inhibition exerted by 1-O-octadeyl-2-O-methyl-sn-glycero-3-phosphocholine and U-73122 (1-[6((17 beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H-pyrrole-2,5-dione), which are pharmacological inhibitors of phosphoinositide-specific phospholipase C. In contrast, when HL-60 cells were induced to differentiate along the granulocytic lineage by dimethyl sulfoxide, we observed a rise in the nuclear DAG mass, which was sensitive to either neomycin or propranolol, two compounds with inhibitory effect on phospholipase D (PLD)-mediated DAG generation. In nuclei of dimethyl sulfoxide-treated HL-60 cells, we observed a rise in the amount of a 90-kDa PLD, distinct from PLD1 or PLD2. When a phosphatidylinositol (4,5) bisphosphate-derived DAG pool was generated in the nucleus, a selective translocation of PKC-beta II occurred. On the other hand, nuclear DAG derived through PLD, recruited PKC-alpha to the nucleus. Both of these PKC isoforms were phosphorylated on serine residues. These results provide support for the proposal that in the HL-60 cell nucleus there are two independently regulated sources of DAG, both of which are capable of acting as the driving force that attracts to this organelle distinct, DAG-dependent PKC isozymes. Our results assume a particular significance in light of the proposed use of pharmacological inhibitors of PKC-dependent biochemical pathways for the therapy of cancer disease.

Microtubule patterning during meiotic maturation in mouse oocytes is determined by cell cycle-specific sorting and redistribution of gamma-tubulin

The topography of microtubule assembly events during meiotic maturation of animal oocytes demands tight spatial control and temporal precision. To better understand what regulates the timing and location of microtubule assembly, synchronously maturing mouse oocytes were evaluated with respect to gamma-tubulin, pericentrin, and total tubulin polymer fractions at specific stages of meiotic progression. gamma-Tubulin remained associated with cytoplasmic centrosomes through diakinesis of meiosis-1. Following chromatin condensation and perinuclear centrosome aggregation, gamma-tubulin relocated to a nuclear lamina-bounded compartment in which meiosis-1 spindle assembly occurred. gamma-Tubulin was stably associated with the meiotic spindle from prometaphase-1 through to anaphase-2, but also exhibited cell cycle-specific relocalization to cytoplasmic centrosomes. Specifically, anaphase onset of both meiosis-1 and -2 was characterized by the concomitant appearance of gamma-tubulin and microtubule nucleation in subcortical centrosomes. Brief pulses of taxol applied at specific cell cycle stages enhanced detection of gamma-tubulin compartmentalization, consistent with a gamma-tubulin localization-dependent spatial restriction of microtubule assembly during meiotic progression. In addition, a taxol pulse during meiotic resumption impaired subsequent gamma-tubulin sorting, resulting in monopolar spindle formation and cell cycle arrest in meiosis-1; despite cell cycle arrest, polar body extrusion occurred roughly on schedule. Therefore, sorting of gamma-tubulin is involved in both the timing of location of meiotic spindle assembly as well as the coordination of karyokinesis and cytokinesis in mouse oocytes.

Re-examination of the mechanisms regulating nuclear inositol lipid metabolism

Although inositol lipids constitute only a very minor proportion of total cellular lipids, they have received immense attention by scientists since it was discovered that they play key roles in a wide range of important cellular processes. In the late 1980s, it was suggested that these lipids are also present within the cell nucleus. Albeit the early reports about the intranuclear localization of phosphoinositides were met by skepticism and disbelief, compelling evidence has subsequently been accumulated convincingly showing that a phosphoinositide cycle is present at the nuclear level and may be activated in response to stimuli that do not activate the inositol lipid metabolism localized at the plasma membrane. Very recently, intriguing new data have highlighted that some of the mechanisms regulating nuclear inositol lipid metabolism differ in a substantial way from those operating at the cell periphery. Here, we provide an overview of recent findings regarding the regulation of both nuclear phosphatidylinositol 3-kinase and phosphoinositide-specific phospholipase C-beta1.