Profile of protein kinase C isozymes and their possible role in mammalian egg activation

Activation of protein kinase C suppresses fragmentation of pig oocytes aged in vitro

When cultured for an extended time, pig oocytes that matured in vitro to the stage of metaphase II undergo the complex process designated as ageing. Under our conditions, some pig oocytes aged 3 days remained at the stage of metaphase II (22%), but others underwent spontaneous parthenogenetic activation (45%), and still others perished through fragmentation (28%) or lysis (5%). Activation of protein kinases C (PKCs) using phorbol-12-myristate-13-acetate (PMA) protects oocytes from fragmentation. None of the oocytes were fragmented after 3 days of aging in 50 nM of PMA. A similar effect (8% of fragmented oocytes) was observed after a 3-day treatment of aging oocytes with 100 μM of 1-stearoyl-2arachidonoyl-sn-glycerol (STEAR). PMA and STEAR activate both calcium-dependent and calcium-independent PKCs. This combined effect on PKCs seems to be essential for the protection of oocytes from fragmentation. Neither the specific activator of calcium-dependent PKCs 1-oleoyl-2-acetyl-sn-glycerol (OLE) nor the specific activator of calcium-independent PKCs dipalmitoyl-l-α-phosphatidylinositol-3,4,5-triphosphate heptaammonium salt (DIPALM) suppressed the fragmentation of aging pig oocytes. Twenty-one percentage of oocytes fragmented when aged for 3 days in 10 μM OLE and 26% of aged oocytes fragmented in 100 nM of DIPALM. However, fragmentation was significantly suppressed to 7% when the oocytes were exposed to the combination of both 10 μM OLE and 100 nM DIPALM. Aging pig oocytes cultured for 1 day with PMA maintained a high capability of being parthenogenetically activated (86% of activated oocytes), using calcium ionophore with 6-dimethylaminopurine. Ageing oocytes treated with PMA also had high capability of cleavage (82%) after their artificial parthenogenetic activation. However, their ability to develop to the stage of blastocyst (12%) was suppressed when compared with oocytes activated immediately after their maturation (29%).

Involvement of PKCζ and GSK3β in the stability of the metaphase spindle

In the somatic cell, the mitotic spindle apparatus is centrosomal, and several isoforms of protein kinase C (PKC) have been associated with the mitotic spindle, but their role in stabilizing the mitotic spindle is still unclear. Other protein kinases such as, glycogen synthase kinase 3β (GSK3β) have also been shown to be associated with the mitotic spindle apparatus. In this study, we show the enrichment of active (phosphorylated) PKCζ at the centrosomal region of the spindle apparatus in metaphase stage of 3T3 cells. In order to understand whether the two kinases PKC and GSK3β are associated with the mitotic spindle, first, the co-localization of phosphorylated PKC isoforms with GSK3β was studied at the poles in metaphase cells. Fluorescence resonance energy transfer (FRET) analysis was used to demonstrate close molecular proximity of phospho-PKCζ with phospho(ser9)GSK3β. Second, the involvement of inactive GSK3β in maintaining an intact mitotic spindle in 3T3 cells was shown. Third, this study also showed that addition of a phospho-PKCζ specific inhibitor to cells can disrupt the mitotic spindle microtubules and some of the proteins associated with it. The mitotic spindle at metaphase in mouse fibroblasts appears to be maintained by PKCζ acting through GSK3β. Phospho-PKCζ is in close molecular proximity to GSK3β, whereas the other isoforms of PKC such as pPKCβII, pPKCγ, pPKCμ, and pPKCθ are not close enough to have significant FRET readings. The close molecular proximity supports the idea that GSK3β may be a substrate of PKCζ.

Rat eggs cannot wait: Spontaneous exit from meiotic metaphase-II arrest

Mammalian eggs await fertilisation while arrested at the second metaphase stage of meiotic division. A network of signalling pathways enables the establishment and maintenance of this metaphase-II arrest. In the absence of fertilisation, mammalian eggs can spontaneously exit metaphase II when parthenogenetically stimulated, or sometimes without any obvious stimulation. Ovulated rat eggs abortively release from metaphase-II arrest once removed from egg donors. Spontaneously activated rat eggs extrude the second polar body and proceed to the so-called metaphase III-'like' stage, with clumps of condensed chromatin scattered in the egg cytoplasm. It is still unclear what makes rat eggs susceptible to spontaneous activation; however, a vague picture of the signalling pathways involved in the process of spontaneous activation is beginning to emerge. Such cell cycle instability is one of the major reasons why it is more difficult to establish nuclear transfer in the rat. This review examines the known predisposing factors and biochemical mechanisms involved in spontaneous activation. The strategies used to prevent spontaneous metaphase-II release in rat eggs will also be discussed.

Effect of age, GV transfer and modified nucleocytoplasmic ratio on PKCα in mouse oocytes and early embryos

Protein kinase C (PKC) is a family of Ser/Thr protein kinases that can be activated by Ca2+, phospholipid and diacylglycerol. There is evidence that PKC plays key roles in the meiotic maturation and activation of mammalian oocytes. The present study aimed to monitor the effect of age, germinal vesicle (GV) transfer and modified nucleoplasmic ratio on the subcellular distribution profile of PKCα, an important isozyme of PKC, in mouse oocytes undergoing meiotic maturation and following egg activation. Germinal vesicle oocytes were collected from 6–8-week-old and 12-month-old mice. Germinal vesicle-reconstructed oocytes and GV oocytes with one-half or one-third of the original oocyte volume were created using micromanipulation and electrofusion. The subcellular localization of PKCα was detected by immunocytochemistry and laser confocal microscopy. Our study showed that PKCα had a similar location pattern in oocytes and early embryos from young and old mice. PKCα was localized evenly in ooplasm, with weak staining in GV at the GV stage, and present in the entire meiosis II (MII) spindle at the MII stage. In pronuclear and 2-cell embryos, PKCα was concentrated in the nucleus except for the nucleolus. After the GV oocytes were reconstructed, the resultant MII oocytes and embryos showed a similar distribution of PKCα between reconstructed and unreconstructed controls. After one-half or two-thirds of the cytoplasm was removed from the GV oocytes, PKCα still had a similar location pattern in MII oocytes and early embryos from the GV oocytes with modified nucleoplasmic ratio. Our study showed that age, GV transfer and modified nucleocytoplasmic ratio does not affect distribution of PKCα during mouse oocyte maturation, activation, and early embryonic mitosis.

Myristoylated alanine-rich C kinase substrate, but not Ca2+/calmodulin-dependent protein kinase II, is the mediator in cortical granules exocytosis

Sperm–egg fusion induces cortical granules exocytosis (CGE), a process that ensures the block to polyspermy. CGE can be induced independently by either a rise in intracellular calcium concentration or protein kinase C (PKC) activation. We have previously shown that myristoylated alanine-rich C kinase substrate (MARCKS) cross-links filamentous actin (F-actin) and regulates its reorganization. This activity is reduced either by PKC-induced MARCKS phosphorylation (PKC pathway) or by its direct binding to calmodulin (CaM; CaM pathway), both inducing MARCKS translocation, F-actin reorganization, and CGE. Currently, we examine the involvement of Ca2+/CaM-dependent protein kinase II (CaMKII) and MARCKS in promoting CGE and show that PKC pathway can compensate for lack of Ca2+/CaM pathway. Microinjecting eggs with either overexpressed protein or complementary RNA of constitutively active αCaMKII triggered resumption of second meiotic division, but induced CGE of an insignificant magnitude compared with CGE induced by wt αCaMKII. Microinjecting eggs with mutant-unphosphorylatable MARCKS reduced the intensity of 12-O-tetradecanoylphorbol 13-acetate or ionomycin-induced CGE by 50%, indicating that phosphorylation of MARCKS by novel and/or conventional PKCs (n/cPKCs) is a pivotal event associated with CGE. Moreover, we were able to demonstrate cPKCs involvement in ionomycin-induced MARCKS translocation and CGE. These results led us to propose that MARCKS, rather than CaMKII, as a key mediator of CGE.

Effects of PKC ζ on early genome transcription activation in mouse 1-cell stage fertilized eggs

Effects of PKC zeta on the activation of embryonic transcription in 1-cell stage fertilized mouse eggs were explored. The effects of PKC antagonist calphostin C and PKC zeta specific inhibitor on the activation of embryonic early transcription were observed by Western blotting and cell immunofluorescence. PKC activity increased gradually from G1 phase to late G2 phase in mouse 1-cell stage fertilized eggs, and reached a maximum in G2 stage. Calphostin C inhibited PKC activity by about 47% in 1-cell stage fertilized eggs. Calphostin C inhibited early transcription in 1-cell stage fertilized eggs (p < 0.01). PKC zeta-Thr410 in G2 were about 27% and 110% higher than those in G1 phase of 1-cell stage fertilized eggs and MII oocytes, respectively. PKC zeta specific inhibitor can also inhibit early transcription in 1-cell stage fertilized eggs (p < 0.05). The results suggest that PKC zeta participates in early transcription activation in mouse 1-cell stage fertilized eggs.

When a sperm meets an egg: block to polyspermy

Embryonic development is initiated after the fertilizing spermatozoon enters the egg and triggers a series of events known as egg activation. Activation results in an increase in intracellular calcium concentration, cortical granule exocytosis (CGE), cell cycle resumption and recruitment of maternal mRNA. CGE is an evolutionary developed mechanism that causes modification of the zona pellucida to prevent penetration of additional spermatozoa, ensuring successful egg activation and embryo development. The egg CGE is a unique and convenient mammalian model for studying the different proteins participating at the membrane fusion cascade, which, unlike other secretory cells, occurs only once in the egg's lifespan. This article highlights a number of proteins, ascribed to participate in CGE and thus the block to polyspermy. CGE can be triggered either by a calcium dependent pathway, or via protein kinase C (PKC) activation that requires a very low calcium concentration. In a recent study, we suggested that the filamentous actin (F-actin) at the egg's cortex is a dynamic network. It can be maneuvered towards allowing CGE by activated actin associated proteins and/or by activated PKC and its down stream proteins, such as myristoylated alanine-rich C kinase substrate (MARCKS). MARCKS, a protein known to cross-link F-actin in other cell types, was found to be expressed and colocalized with actin in non-activated MII eggs. We further demonstrated MARCKS dissociation from actin after activation by ionomycin, a process that can lead to the breakdown of the actin network, thus allowing CGE. The more we know of the intricate process of CGE and of the proteins participating in it, the more the assisted reproductive procedures might benefit from that knowledge.

Expression profile of protein kinase C isozymes in preimplantation mouse development

In the preimplantation mouse embryo, the protein kinase C (PKC) family has been implicated in regulation of egg activation, progression of meiotic and mitotic cell cycles, embryo compaction, and blastulation, but the involvement of the individual isozymes is largely unknown. Here, using semiquantitative immunocytochemistry and confocal microscopy we analyze the relative amount and subcellular distribution of ten isozymes of PKC (alpha, betaI, betaII, gamma, delta, epsilon, eta, theta, zeta, iota/lambda) and a PKC-anchoring protein, receptor for activated C-kinase 1 (RACK1). Our results show that all of these isoforms of PKC are present between the two-cell and blastocyst stages of mouse preimplantation development, and that each has a distinct, dynamic pattern and level of expression. The data suggest that different complements of the isozymes are involved in various steps of preimplantation development, and will serve as a framework for further functional studies of the individual isozymes. In particular, there was a transient increase in the nuclear concentration of several isozymes at the early four-cell stage, suggesting that some of the PKC isozymes might be involved in regulation of nuclear organization and function in the early mouse embryo.

Subcellular localization of protein kinase C delta and epsilon affects transcriptional and post-transcriptional processes in four-cell mouse embryos

During mouse preimplantation development, two isozymes of protein kinase C (PKC), delta and epsilon, transiently localize to nuclei at the early four-cell stage. In order to study their functions at this stage, we altered the subcellular localization of these isozymes (ratio of nuclear to cytoplasmic concentrations) with peptides that specifically activate or inhibit translocation of each isozyme. The effects of altering nuclear concentration of each isozyme on transcription (5-bromouridine 5'-triphosphate (BrUTP) incorporation), amount and distribution of small nuclear ribonucleoproteins (snRNPs), nucleolar dynamics (immunocytochemistry for Smith antigen (Sm) protein) and the activity of embryonic alkaline phosphatase (EAP; histochemistry) were examined. We found that nuclear concentration of PKC epsilon correlated with total mRNA transcription. Higher nuclear concentrations of both PKC delta and epsilon decreased storage of snRNPs in Cajal bodies and decreased the number of nucleoli, but did not affect the nucleoplasmic concentration of snRNPs. Inhibiting translocation of PKC delta out of the nucleus at the early four-cell stage decreased cytoplasmic EAP activity, whereas inhibiting translocation of PKC epsilon increased EAP activity slightly. These results indicate that translocation of PKC delta and epsilon in and out of nuclei at the early four-cell stage in mice can affect transcription or message processing, and that sequestration of these PKC in nuclei can also affect the activity of a cytoplasmic protein (EAP).

Association between myristoylated alanin-rich C kinase substrate (MARCKS) translocation and cortical granule exocytosis in rat eggs

Cortical granule exocytosis (CGE), following egg activation, is a secretory process that blocks polyspermy and enables successful embryonic development. CGE can be triggered independently by either a rise in intracellular calcium concentration ([Ca2+]i) or activation of protein kinase C (PKC). The present study investigates the signal transduction pathways leading to CGE through activation of PKC or stimulation of a rise in [Ca2+]i. Using Western blot analysis, co-immunoprecipitation and immunohistochemistry, combined with various inhibitors or activators, we investigated the link between myristoylated alanin-rich C kinase substrate (MARCKS) translocation and CGE. We were able to demonstrate translocation of MARCKS from the plasma membrane to the cortex, in fertilized as well as in parthenogenetically activated eggs. MARCKS phosphorylation was demonstrated upon PKC activation, whereas a PKC inhibitor (myrPKCψ) prevented both MARCKS translocation and CGE in 12-O-tetradecanoyl phorbol-13-acetate (TPA)-activated eggs. We have further shown that upon egg activation the amount of phosphorylated MARCKS (p-MARCKS) and the amount of calmodulin bound to MARCKS were increased. MARCKS translocation in ionomycin activated eggs was also inhibited by the calmodulin inhibitor N-(6-aminohexyl)-5-chloro-1-napthalenesulfonamide hydrochloride (W7). These results complement other studies showing MARCKS requirement for exocytosis and imply that upon fertilization, MARCKS translocation is followed by CGE. These findings present a significant contribution to our understanding of CGE in mammalian eggs in particular, as well as cellular exocytosis in general.

Effect of protein kinase C inhibitors on porcine oocyte activation

The effect of protein kinase C (PKC) inhibitors on porcine oocyte activation by calcium ionophore A23187 was studied. Calcium ionophore applied in a 50 microM concentration for 10 min induced activation in 74% of oocytes matured in vitro. When the ionophore-treated oocytes were exposed to the effect of bisindolylmaleimide I, which inhibits calcium-dependent PKC isotypes (PKC-alpha, -beta(I), -beta(II), -gamma,) and calcium-independent PKC isotypes (PKC-delta, -epsilon), the portion of activated oocytes decreased (at a concentration of 100 nM, 2% of the oocytes were activated). Go6976, the inhibitor of calcium-dependent PKC isotypes PKC-alpha, -beta(I) did not prevent the action of the oocytes treated with calcium ionophore in concentrations from 1 to 100 microM. The inhibitor of PKC-beta(I) and beta(II) isotypes, hispidin, in a concentration of 2 microM-2 mM, was not effective either. The inhibitor of PKC-delta isotype, rottlerin, suppressed activation of the oocytes by calcium ionophore (no oocyte was activated at 10 microM concentration). The PKC-delta isotype in matured porcine oocytes, studied by Western blot analysis, appeared as non-truncated PKC-delta of 77.5 kDa molecular weight, on the one hand, and as truncated PKC-delta, which was present in the form of a doublet of approximately 62.5 and 68 kDa molecular weight, on the other hand. On the basis of these results, it can be supposed that PKC participates in the regulation of processes associated with oocyte activation. Calcium-dependent PKC-alpha, -beta isotypes do not seem to play any significant role in calcium activation. The activation seems to depend on the activity of the calcium-independent PKC-delta isoform.

Relative contribution of cell contact pattern, specific PKC isoforms and gap junctional communication in tight junction assembly in the mouse early embryo

In mouse early development, cell contact patterns regulate the spatial organization and segregation of inner cell mass (ICM) and trophectoderm epithelium (TE) during blastocyst morphogenesis. Progressive membrane assembly of tight junctional (TJ) proteins in the differentiating TE during cleavage is upregulated by cell contact asymmetry (outside position) and suppressed within the ICM by cell contact symmetry (inside position). This is reversible, and immunosurgical isolation of the ICM induces upregulation of TJ assembly in a sequence that broadly mimics that occurring during blastocyst formation. The mechanism relating cell contact pattern and TJ assembly was investigated in the ICM model with respect to PKC-mediated signaling and gap junctional communication. Our results indicate that complete cell contact asymmetry is required for TJ biogenesis and acts upstream of PKC-mediated signaling. Specific inhibition of two PKC isoforms, PKCdelta and zeta, revealed that both PKC activities are required for membrane assembly of ZO-2 TJ protein, while only PKCzeta activity is involved in regulating ZO-1alpha+ membrane assembly, suggesting different mechanisms for individual TJ proteins. Gap junctional communication had no apparent influence on either TJ formation or PKC signaling but was itself affected by changes of cell contact patterns. Our data suggest that the dynamics of cell contact patterns coordinate the spatial organization of TJ formation via specific PKC signaling pathways during blastocyst biogenesis.

Translocation of phospho-protein kinase Cs implies their roles in meiotic-spindle organization, polar-body emission and nuclear activity in mouse eggs

The protein kinase Cs (PKCs) are a family of Ser/Thr protein kinases categorized into three subfamilies: classical, novel, and atypical. The phosphorylation of PKC in germ cells is not well defined. In this study, we described the subcellular localization of phopho-PKC in the process of mouse oocyte maturation, fertilization, and early embryonic mitosis. Confocal microscopy revealed that phospho-PKC (pan) was distributed abundantly in the nucleus at the germinal vesicle stage. After germinal vesicle breakdown, phospho-PKC was localized in the vicinity of the condensed chromosomes, distributed in the whole meiotic spindle, and concentrated at the spindle poles. After metaphase I, phospho-PKC was translocated gradually to the spindle mid-zone during emission of the first polar body. After sperm penetration and electrical activation, the distribution of phospho-PKC was moved from the spindle poles to the spindle mid-zone. After the extrusion of the second polar body (PB2) phospho-PKC was localized in the area between the oocyte and the PB2. In fertilized eggs, phospho-PKC was concentrated in the pronuclei except for the nucleolus. Phospho-PKC was dispersed after pronuclear envelope breakdown, but distributed on the entire spindle at mitotic metaphase. The results suggest that PKC activation may play important roles in regulating spindle organization and stabilization, polar-body extrusion, and nuclear activity during mouse oocyte meiosis, fertilization, and early embryonic mitosis.

The involvement of protein kinase C and actin filaments in cortical granule exocytosis in the rat

Mammalian sperm–egg fusion results in cortical granule exocytosis (CGE) and resumption of meiosis. Studies of various exocytotic cells suggest that filamentous actin (F-actin) blocks exocytosis by excluding secretory vesicles from the plasma membrane. However, the exact function of these microfilaments, in mammalian egg CGE, is still elusive. In the present study we investigated the role of actin in the process of CGE, and the possible interaction between actin and protein kinase C (PKC), by using coimmunoprecipitation, immunohistochemistry and confocal microscopy. We identified an interaction between actin and the PKC alpha isoenzyme in non-activated metaphase II (MII) eggs and in eggs activated by phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA). F-actin was evenly distributed throughout the egg’s cytosol with a marked concentration at the cortex and at the plasma membrane. A decrease in the fluorescence signal of F-actin, which represents its depolymerization/reorganization, was detected upon fertilization and upon parthenogenetic activation. Exposing the eggs to drugs that cause either polymerization or depolymerization of actin (jasplakinolide (JAS) and cytochalasin D (CD) respectively) did not induce or prevent CGE. However, CD, but not JAS, followed by a low dose of TPA doubled the percentage of eggs undergoing complete CGE, as compared with TPA alone. We further demonstrated that myristoylated alanin-rich C kinase substrate (MARCKS), a protein known to cross-link F-actin in other cell types, is expressed in rat eggs and is colocalized with actin. In view of our results, we suggest that the cytoskeletal cortex is not a mere physical barrier that blocks CGE, but rather a dynamic network that can be maneuvered towards allowing CGE by activated actin-associated proteins and/or by activated PKC.

Altered protein kinase C activation associated with rat embryonic dysmorphogenesis

It has been suggested that protein kinase C (PKC) is involved in the etiology of diabetic complications. The aim of the present study was to investigate the putative involvement of different PKC isoforms (alpha, beta1, beta 2, gamma, delta, epsilon, and zeta) in the embryopathy of diabetic rat pregnancy. Embryos were collected from normal and diabetic rats and assayed for PKC activity, PKC mRNA levels, and PKC protein distribution on gestational d 10 and 11. Embryos of diabetic rats showed markers of increased activity of PKC-alpha, PKC-beta1, PKC-gamma, PKC-delta, and PKC-zeta compared with embryos of normal rats on d 10. In addition, the malformed embryos had further increased PKC-gamma, and PKC-delta activity markers compared with nonmalformed embryos of diabetic rats on gestational d 10. In contrast, maternal diabetes caused only two alterations in PKC activity markers on gestational d 11, i.e. both PKC-alpha and PKC-zeta were decreased in embryos of diabetic rats. We found increased mRNA levels of PKC-beta 1 and PKC-zeta on d 10 in embryos of diabetic rats and decreased mRNA levels of PKC-gamma on d 11 in embryos of diabetic rats. Malformed embryos from diabetic rats showed increased distribution of PKC-beta 1 and PKC-beta 2 protein in the tissue compared with nonmalformed embryos from diabetic rats and embryos from normal rats. We conclude that diabetic rat embryopathy may be associated with increased activity and enhanced tissue distribution of several PKC isoforms in early organogenesis.

PKC signaling at fertilization in mammalian eggs

Protein kinase C (PKC) has been proposed to regulate major egg activation events during mammalian fertilization. Most of the evidence supporting this assumption has first been obtained using pharmacological activation and inhibition of the kinase, while egg activation was assessed by checking for exocytosis of the cortical granules, extrusion of the second polar body and formation of pronuclei. However, results have been inconclusive and sometimes contradictory regarding the exact role of PKC in regulating egg activation events. The PKC family is composed of various isotypes, which differ in their modular structures and regulatory properties. Hence the need to re-examine the roles of egg PKCs more specifically. Mammalian eggs express many PKC isotypes, the roles of which have been investigated using immunodetection, isotype-specific inhibition and, more recently, live imaging of fluorescent chimaeras. Here, I review the recent development of PKC research in mammalian fertilization and the evidence for a specific role for certain PKC isotypes in fertilization-induced egg activation.

PKC isotypes in post-activated and fertilized mouse eggs: association with the meiotic spindle

Several isotypes of protein kinase C (PKC) have been reported to be expressed in mammalian eggs, but it is unknown whether these isotypes have a common function in the egg during or within the first few hours of fertilization. Here we show that the isotypes of PKC exhibit distinct patterns of enrichment immediately after mouse egg activation. PKCalpha and gamma accumulate in the egg cortex 25 min post-activation, while only PKCalpha accumulates at the contractile ring of the forming second polar body about 1.5 h post-activation. PKCzeta exhibits some unique features that resulted in it being the focus of more extensive analysis. PKCzeta is tightly associated with the meiotic spindle as determined by detergent extraction and is closely associated with alpha-tubulin as determined by FRET analysis in the metaphase II (MII) egg. In addition, after egg activation, PKCzeta remains associated with the spindle as it transits into anaphase II and later telophase II, becoming associated with the midzone microtubules. Antibodies to the active form of PKCzeta are enriched on the spindle poles and later in development on the midzone microtubules. Active PKCzeta also is enriched in both pronuclei in the 6-h post-fertilization and in the 14-h post-fertilization embryo as well as in the nuclei of the two-cell embryo. Inhibition of PKCzeta, but not inhibition of other isotypes of PKC, results in rapid disruption of the meiotic spindle. This study suggests that PKCzeta has a role in spindle stability, while other PKC isotypes have different roles in the conversion of the egg to the zygote.

Specific PKC isoforms regulate blastocoel formation during mouse preimplantation development

During early mammalian development, blastocyst morphogenesis is achieved by epithelial differentiation of trophectoderm (TE) and its segregation from the inner cell mass (ICM). Two major interrelated features of TE differentiation required for blastocoel formation include intercellular junction biogenesis and a directed ion transport system, mediated by Na+/K+ ATPase. We have examined the relative contribution of intercellular signalling mediated by protein kinase C (PKC) and gap junctional communication in TE differentiation and blastocyst cavitation. The distribution pattern of four (delta, theta, iota/lambda, zeta) PKC isoforms and PKCmicro/PKD1 showed partial colocalisation with the tight junction marker ZO-1alpha+ in TE and all four PKCs (delta, theta, iota/lambda, zeta) showed distinct TE/ICM staining patterns (predominantly at the cell membrane within the TE and cytoplasmic within the ICM), indicating their potential contribution to TE differentiation and blastocyst morphogenesis. Specific inhibition of PKCdelta and zeta activity significantly delayed blastocyst formation. Although modulation of these PKC isoforms failed to influence the already established programme of epithelial junctional differentiation within the TE, Na+/K+ ATPase alpha1 subunit was internalised from membrane to cytoplasm. Inhibition of gap junctional communication, in contrast, had no influence on any of these processes. Our results demonstrate for the first time that distinct PKC isotypes contribute to the regulation of cavitation in preimplantation embryos via target proteins including Na+/K+ ATPase.

PKC signalling regulates tight junction membrane assembly in the pre-implantation mouse embryo

Epithelial differentiation including tight junction (TJ) formation occurs exclusively within the trophectoderm (TE) lineage of the mouse blastocyst. Here we examine mechanisms by which TJ protein membrane assembly might be regulated by protein kinase C (PKC) in the embryo. To overcome the inherent staging asynchrony of individual blastomeres within intact embryos, we have used isolated inner cell masses (ICMs) from early blastocysts to induce epithelial differentiation in their outer cells responding to their new cell contact pattern. Two TJ proteins examined retain their order of membrane assembly in isolated ICMs in culture as during normal development (early-assembling ZO-2 and late-assembling ZO-1α+), but this process is highly accelerated. Using six chemical modulators of PKC activity, we show here that PKC signalling is involved in the regulation of TJ membrane assembly. While indolactam-mediated PKC activation stimulates membrane assembly of both TJ proteins, TPA-mediated PKC activation stimulates only that of ZO-1α+. The PKC inhibitors Ro-31-8220, Ro-31-8425 and Gö 6983 suppress the stimulatory effect of both PKC activators on membrane assembly to varying extents according to inhibitor and TJ protein examined. Gö 6983 similarly inhibits ZO-2 and ZO-1α+membrane assembly. PKC inhibition by Gö 6976 appeared to stimulate TJ membrane assembly. Despite the broad PKC isotype specificity of the inhibitors used, these data suggest that the two TJ proteins are differently regulated by PKC isotypes or subfamilies. As Gö 6983 uniquely affects aPKC (particularly PKCζ) and we find that both PKCδ and ζ relocate upon activator treatment to colocalise partially with the TJ proteins in isolated ICMs, we suggest that at least PKCδ and ζ may play a central role in regulating TJ membrane assembly.

Regulation of cleavage by protein kinase C inChaetopterus

We report that protein kinase C (PKC) plays a regulatory role in early cleavage in Chaetopterus eggs. Using Western blotting, we assayed the expression patterns of conventional PKCs (cPKC), novel PKCs (nPKC), and atypical PKCs (aPKC). During early development after fertilization, PKC protein levels varied independently by isoform. PKC protein expression during differentiation, without cleavage and after parthenogenetic activation, was very similar to that during normal development indicating that PKC gene expression does not require cellularization. Since PKC has been shown to regulate meiosis in this organism, we also assayed the membrane association of these isoforms as an indicator of their activation during meiosis and early cleavage. PKC-gamma transiently associated with membranes and therefore became activated before meiotic division and cleavage, whereas PKC-alpha and -beta transiently dissociated from membranes and therefore became inactivated at these times. Inhibition of these PKC isoforms by bisindolylmaleimide I had no effect on cleavage or early development to the trochophore larva, indicating that PKC-gamma activation is not essential for cleavage or early development. However, their persistent activation by thymeleatoxin blocked cleavage. The results indicate that the dissociation of PKC-alpha and/or -beta from the membrane fraction, and therefore their inactivation, is essential for normal cleavage. Elevated PKC activity is essential for nuclear envelope breakdown and spindle formation at meiosis I. By contrast, down-regulation of this activity is essential for cleavage after fertilization.

Cellular and molecular mechanisms leading to cortical reaction and polyspermy block in mammalian eggs

Following fusion of sperm and egg, the contents of cortical granules (CG), a kind of special organelle in the egg, release into the perivitelline space (cortical reaction), causing the zona pellucida to become refractory to sperm binding and penetration (zona reaction). Accumulating evidence demonstrates that mammalian cortical reaction is probably mediated by activation of the inositol phosphate (PIP(2)) cascade. The sperm-egg fusion, mediated by GTP-binding protein (G-protein), may elicit the generation of two second messengers, inositol 1,4,5 triphosphate (IP(3)) and diacylglycerol (DAG). The former induces Ca(2+) release from intracellular stores and the latter activates protein kinase C (PKC), leading to CG exocytosis. Calmodulin-dependent kinase II (CaMKII) may act as a switch in the transduction of the calcium signal. The CG exudates cause zona sperm receptor modification and zona hardening, and thus block polyspermic penetration. Oolemma modification after sperm-egg fusion and formation of CG envelope following cortical reaction also contribute to polyspermy block.

Cadherins expression during gamete maturation and fertilization in the rat

A role for adhesion molecules in gamete fusion, preceding fertilization, has been previously suggested. We investigated the presence of cadherins, Ca(2+) dependent cell-cell adhesion molecules, in rat oocytes and spermatozoa using an anti-pan-cadherin antibody and specific antibodies against the 3 classical cadherins: E- (epithelial), P- (placental), and N- (neural) cadherins. Electrophoretic separation was performed on samples of lysed oocytes of different stages: germinal vesicle oocytes, metaphase II eggs, newly fertilized and 2-cell embryos, as well as spermatozoa from testes, caput and cauda epididymis and ejaculate. Localization of cadherins was determined on intact, gametes by immunocytochemistry, using confocal microscopy. Immunoblotting with the pan-cadherin antibody revealed a major band of approximately 120 kD in all oocyte and sperm extracts. Oocytes presented E-cadherin at appropriate molecular weight but N-cadherin only as a specific 40 kD band. In sperm lysate, at all stages, both E- and N-cadherin were demonstrated as major protein bands but a series of lower molecular weight proteins (that may represent protein degradation) were also detected. Immunohistochemical evaluation showed that E- and N-cadherins are already present on the plasma membrane of immature unfertilized oocytes, although their concentration increases after fertilization in early cleavage stage embryos. Cadherin localization on spermatozoa changed during maturation from a dispersed pattern over the entire head plasma membrane of testicular spermatozoa to a restricted equatorial and post-acrosomal plasma membrane staining in ejaculated spermatozoa. These findings suggest a specific cadherin organization at the fusogenic domains of both gametes.

Translocation of the classic protein kinase C isoforms in porcine oocytes: implications of protein kinase C involvement in the regulation of nuclear activity and cortical granule exocytosis

Protein kinase C (PKC) is a family of Ser/Thr protein kinases categorized into three subfamilies: classical, novel, and atypical. The subcellular localization of classical PKCalpha, -betaI, and -gamma in the process of porcine oocyte maturation, fertilization, and parthenogenetic activation and their involvement in cortical granule (CG) exocytosis were investigated. The results of Western blot showed that PKCalpha, -betaI, and -gamma were expressed in the oocytes at the germinal vesicle (GV) and metaphase II (MII) stages. Confocal microscopy revealed that the three PKC isoforms were concentrated in the GV but evenly distributed in the cytoplasm of MII eggs. PKCalpha and -gamma were translocated to the plasma membrane soon after sperm penetration. cPKCs migrated into the pronucleus in fertilized eggs. Following treatment with a PKC activator, phorbol 12-myristate 13-acetate (PMA), CGs were released and PKCalpha and -gamma were translocated to the membrane. The CG exocytosis and PKC redistribution induced by PMA could be blocked by the PKC inhibitor staurosporine. Parthenogenetic stimulation with ionophore A23187 or electrical pulse also induced cPKC translocation and CG exocytosis. Eggs injected with PKCalpha isoform-specific antibody failed to undergo CG exocytosis after PMA treatment or fertilization. The results suggest that cPKCs, especially the alpha-isotype, regulate nuclear function and CG exocytosis in porcine eggs.

A role for protein kinase C during rat egg activation

Upon sperm-egg interaction, an increase in intracellular calcium concentration ([Ca(2+)](i)) is observed. Several studies reported that cortical reaction (CR) can be triggered not only by a [Ca(2+)](i) rise but also by protein kinase C (PKC) activation. Because the CR is regarded as a Ca(2+)-dependent exocytotic process and because the calcium-dependent conventional PKCs (cPKC) alpha and beta II are considered as exocytosis mediators in various cell systems, we chose to study activation of the cPKC in the rat egg during in vivo fertilization and parthenogenetic activation. By using immunohistochemistry and confocal microscopy techniques, we demonstrated, for the first time, the activation of the cPKC alpha, beta I, and beta II during in vivo fertilization. All three isozymes examined presented translocation to the egg's plasma membrane as early as the sperm-binding stage. However, the kinetics of their translocation was not identical. Activation of cPKC alpha was obtained by the phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA) or by 1-oleoyl-2-acetylglycerol (OAG) but not by the calcium ionophore ionomycin. PKC alpha translocation was first detected 5-10 min after exposure to TPA and reached a maximum at 20 min, whereas in eggs activated by OAG, translocation of PKC alpha was observed almost immediately and reached a maximum within 5 min. These results suggest that, although [Ca(2+)](i) elevation on its own does not activate PKC alpha, it may accelerate OAG-induced PKC alpha activation. We also demonstrate a successful inhibition of the CR by a myristoylated PKC pseudosubstrate (myrPKCPsi), a specific PKC inhibitor. Our study suggests that exocytosis can be triggered independently either by a [Ca(2+)](i) rise or by PKC.

Signalling in mammalian egg activation: role of protein kinases

Embryonic development is initiated after the fertilizing spermatozoon enters the egg and triggers a process known as 'egg activation'. Activation results in an increase in intracellular calcium concentration, cortical granule exocytosis (CGE), cell cycle resumption and recruitment of maternal mRNA. Various treatments can induce parthenogenetic activation characterized by the same manifestations. Signal transduction pathways similar to those known for somatic cells mediate the mammalian egg activation. This review focuses on the signal transduction pathways that occur during mammalian fertilization and during parthenogenetic egg activation. We discuss the possibility that members of the protein tyrosine kinase (PTKs) families, the Src family PTKs in particular, operate during egg activation and that protein kinase C can induce CGE.

Signaling through protein kinases during egg activation

Following penetration of the zona pellucida, the mammalian spermatozoon binds and fuses with the egg plasma membrane, thereby fertilizing the egg that is still arrested at the second metaphase. Fertilization initiates in the egg a sequence of events referred to as 'egg activation'. An initial increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) appears to be the very early cellular event observed which leads to the cortical granules exocytosis and resumption of meiosis. Various treatments can induce parthenogenetic activation mimicking at least part of the fertilization events. Similar to somatic cells, studies in mammalian eggs suggest that signal transduction pathways mediate egg activation. The initial increase in [Ca(2+)](i) appears to be critical for egg activation. However, other messengers such as protein kinase C (PKC) and protein tyrosine kinases (PTKs), were suggested as possible inducers of some aspects of egg activation. In the present work, studies concerning the involvement of protein kinases during egg activation in our laboratory and in others are summarized.

Identification of a translocation deficiency in cortical granule secretion in preovulatory mouse oocytes

Preovulatory, germinal vesicle (GV)-stage mouse oocytes are unable to undergo normal cortical granule (CG) secretion. Full secretory competence is observed by metaphase II (MII) of meiosis and involves the development of calcium response mechanisms. To identify the deficient or inhibited step in CG secretion, preovulatory GV-stage oocytes were stimulated and tested for their ability to undergo translocation, docking, and/or fusion. The mean CG distance to the plasma membrane was not reduced in fertilized or sperm fraction-injected, GV-stage oocytes relative to that in control GV-stage oocytes. In addition, analysis of individual CG distances to the plasma membrane indicated no subpopulation of CGs competent to translocate. Further analysis demonstrated that secretory incompetence likely is not due to a lack of proximity of CGs to the egg's primary calcium store, the endoplasmic reticulum. Calcium/calmodulin-dependent protein kinase II (CaMKII), which is reportedly involved in secretory granule translocation and secretion in many cells, including eggs, was investigated. A 60-kDa CaMKII isoform detected by Western blot analysis increased 150% during oocyte maturation. The CaMKII activity assays indicated that MII-stage eggs correspondingly have 110% more maximal activity than GV-stage oocytes. These data demonstrate that the primary secretory deficiency is due to a failure of CG translocation, and that a maturation-associated increase in CaMKII correlates with the acquisition of secretory competence and the ability of the egg to undergo normal activation.

Phosphorylation of mitogen-activated protein kinase is regulated by protein kinase C, cyclic 3',5'-adenosine monophosphate, and protein phosphatase modulators during meiosis resumption in rat oocytes

Mitogen-activated protein (MAP) kinase, protein kinase C (PKC), cAMP, and okadaic acid (OA)-sensitive protein phosphatases (PPs) have been suggested to be involved in oocyte meiotic resumption. However, whether these protein kinases and phosphatases act by independent pathways or interact with each other in regulating meiosis resumption is unknown. In the present study, we aimed to determine the regulation of meiosis resumption and MAP kinase phosphorylation by PKC, cAMP, and OA-sensitive PPs in rat oocytes using an in vitro oocyte maturation system and Western blot analysis. We found that ERK1 and ERK2 isoforms of MAP kinases existed in a dephosphorylated (inactive) form in germinal vesicle breakdown (GVBD)-incompetent and GVBD-competent germinal vesicle intact (GVI) oocytes as well as GVBD oocytes at equivalent levels. These results indicate that MAP kinases are not responsible for the initiation of normal meiotic resumption in rat oocytes. However, when GVBD-incompetent and GVBD-competent oocytes were incubated in vitro for 5 h, MAP kinases were phosphorylated (activated) in GVBD-competent oocytes, but not in meiotic-incompetent oocytes, suggesting that oocytes acquire the ability to phosphorylate MAP kinase during acquisition of meiotic competence. We also found that both meiosis resumption and MAP kinase phosphorylation were inhibited by PKC activation or cAMP elevation. Moreover, these inhibitory effects were overcome by OA, which inhibited PP1/PP2A activities. These results suggest that both cAMP elevation and PKC activation inhibit meiosis resumption and MAP kinase phosphorylation at a step prior to OA-sensitive protein phosphatases. In addition, inhibitory effects of cAMP elevation on meiotic resumption and MAP kinase phosphorylation were not reversed by calphostin C-induced PKC inactivation, indicating that cAMP inhibits both meiotic resumption and MAP kinase activation in a PKC-independent manner.

Molecular and biochemical regulation of early mammalian development

Fertilization initiates a rapid series of changes that restructures the egg into the zygote and initiates the program of early development. These changes in the cell occur while the genetic complement of the egg and sperm are in a highly condensed state and unable to participate in transcription. The egg cytoplasm, formed by the maternal genome, contains the necessary components that mediate the early restructuring of egg into zygote. These changes are mediated by a series of cytoplasmic signal transduction events initiated by the rise in [Ca2+]i caused when the sperm penetrates the egg. The structural changes that the egg undergoes are rapid and result in the extensive remodeling of this specialized cell. Protein kinase C (PKC) and calcium/calmodulin-dependent protein kinase II (CaM KII) are two pivotal signaling agents that mediate several of these rapid modifications in cell structure. Studies indicate the meiotic spindle serves as an architectural element in the egg that acts to colocalize elements from several of the key signaling pathways and may provide a means for these pathways to interact. In mammals, transcription begins earlier than in zygotes from other classes of organisms, starting several hours after fertilization in the male and female pronuclei and continuing in the embryonic nuclei. Studies indicate that nuclei undergo an initial state that is permissive for transcription, and then in Gap 2 of the two-cell embryo, enter a transcriptionally repressive state. These changes have been linked to the times during the cell cycle when the DNA is replicated, and also have been proposed as a requirement for proper initiation of the program of early development.

The expression and stage-specific localization of protein kinase C isotypes during mouse preimplantation development

Signaling events mediate many processes that act during embryogenesis to initiate the program of early development. Within the cell many of these changes are mediated through the activation or inactivation of kinases and phosphatases. Protein kinase C (PKC) is one kinase that has been shown to be involved in at least two developmental transitions during early development, fertilization and embryonic compaction. PKC is a family of kinases whose various isotypes have differing requirements for activation of the kinase that include the availability of calcium, diacylglycerol, and negatively charged phospholipids. The presence of more than one isotype in an egg or blastomere of the embryo would provide the possibility that different isotypes mediate distinct signaling pathways in the cells. To address this possibility the different isotypes of PKC were examined at the mRNA and protein levels during preimplantation development in the mouse. Our results demonstrate that seven isotypes of PKC are present during preimplantation development in mouse, some are of maternal origin and others appear after fertilization. Two isotypes have a stage-dependent nuclear localization. In addition, within each blastomere PKC isotypes occupy different subcellular locations in a stage-dependent fashion.

Molecular cloning of rat sperm galactosyl receptor, a C-type lectin with in vitro egg binding activity

Rat sperm galactosyl receptor is a member of the C-type animal lectin family showing preferential binding to N-acetylgalactosamine compared to galactose. Binding is mediated by a Ca(2+)-dependent carbohydrate-recognition domain (CRD) identical to that of the minor variant of rat hepatic lectin receptor 2/3 (RHL-2/3). The molecular organization of the genomic DNA, cDNA, and derived amino acid sequence of rat testis galactosyl receptor have been determined and in vitro fertilization studies were conducted to ascertain its role. We have determined that the rat testis galactosyl receptor gene generates two mRNA species: one species, designated liver-type, is identical to RHL-2/3; the other, designated testis-type, contains one unspliced intron (86 nt) which alters the reading frame and changes the amino acid sequence of the carboxyl terminus. As a result, the CRD (glutamine-proline-aspartic acid/QPD) and flanked Ca(2+)-binding amino acid sequences were not present in the testis-type protein. Northern and Southern blots demonstrated presence of transcripts with unspliced intron in rat sperm but not liver. Similarly, antibody, raised against a synthetic 12-amino acid peptide (p12) encoded by the unspliced intron, recognized in immunoblots a 54 kDa receptor protein in protein extracts from testis but not from liver. Immunofluorescence and immunogold electron microscopy studies demonstrated that both protein species localized on the plasma membrane surface of the head and tail of rat sperm. Furthermore, capacitated rat sperm preincubated with polyclonal antisera to RHL-2/3 or to the CRD of the liver-type galactosyl receptor showed a statistically significant decrease in the in vitro fertilization rate. We conclude that rat sperm galactosyl receptor may play a role in egg binding and that an undetermined molecular mechanism operates to generate two proteins with identical intracellular amino terminal domain but only one of them displays a CRD and associated Ca(2+)-binding sites at the carboxyl terminal extracellular domain.

Fertilization signalling and protein-tyrosine kinases

Fertilization is initiated by species-specific gamete cell recognition, i.e. sperm-egg interaction, followed by a rapid and sustained activation of multiple cellular and biochemical events, collectively called 'egg activation', which is indispensable for successful formation of zygotic nucleus and later embryogenesis. It is well known that sperm-induced egg activation is mediated by a transient release of calcium ions that originates from the sperm entry point and propagates through the entire egg cytoplasm. It is unclear, however, what kind of upstream events prelude to the calcium transient after sperm-egg interaction. Recently, much attention has been paid to the role of protein-tyrosine phosphorylation in egg activation process by a number of studies on some well-established model organisms. These includes marine invertebrates, frogs, and mammals. In this review, we will summarize the recent findings that begin to uncover a 'missing link' between sperm-egg interaction and egg activation with emphasis on the role of egg protein-tyrosine kinases (PTKs) in Xenopus egg fertilization.