Conventional PKCs regulate the temporal pattern of Ca2+ oscillations at fertilization in mouse eggs

Activation of fertilized and nuclear transfer eggs

In all animal species, initiation of embryonic development occurs shortly after the joining together of the gametes from each of the sexes. The first of these steps, referred to as "egg activation", is a series of molecular events that results in the syngamy of the two haploid genomes and the beginning of cellular divisions for the new diploid embryo. For many years it has been known that the incoming sperm drives this process, as an unfertilized egg will remain dormant until it can no longer sustain normal metabolic processes. Until recently, it was also believed that the sperm was the only cell capable of creating a viable embryo and offspring. Recent advances in cell biology have allowed researchers to not only understand the molecular mechanisms of egg activation, but to exploit the use of pharmacological agents to bypass sperm-induced egg activation for the creation of animals by somatic cell nuclear transfer. This chapter will focus on the molecular events of egg activation in mammals as they take place during fertilization, and will discuss how these mechanisms are successfully bypassed in processes such as somatic cell nuclear transfer.

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.

Calcium oscillations and mammalian egg activation

Fertilization in all species studied to date induces an increase in the intracellular concentration of free calcium ions ([Ca2+]i) within the egg. In mammals, this [Ca2+]i signal is delivered in the form of long-lasting [Ca2+]i oscillations that begin shortly after fusion of the gametes and persist beyond the time of completion of meiosis. While not fully elucidated, recent evidence supports the notion that the sperm delivers into the ooplasm a trigger of oscillations, the so-called sperm factor (SF). The recent discovery that mammalian sperm harbor a specific phospholipase C (PLC), PLCzeta has consolidated this view. The fertilizing sperm, and presumably PLCzeta promote Ca2+ release in eggs via the production of inositol 1,4,5-trisphosphate (IP3), which binds and gates its receptor, the type-1 IP3 receptor, located on the endoplasmic reticulum, the Ca2+ store of the cell. Repetitive Ca2+ release in this manner results in a positive cumulative effect on downstream signaling molecules that are responsible for the completion of all the events comprising egg activation. This review will discuss recent advances in our understanding of how [Ca2+]i oscillations are initiated and regulated in mammals, highlight areas of discrepancies, and emphasize the need to better characterize the downstream molecular cascades that are dependent on [Ca2+]i oscillations and that may impact embryo development.

Expression of a fluorescent recombinant form of sperm protein phospholipase C zeta in mouse epididymal sperm by in vivo gene transfer into the testis

OBJECTIVE

To use in vivo gene transfer into the testis by electroporation to express a fluorescent recombinant form of a testis-specific gene in the mature epididymal sperm of mice and thus study the pattern of gene localization.

DESIGN

Controlled animal study.

SETTING

Research laboratory at the University of Oxford.

ANIMAL(S)

Four- to 6-week-old male mice.

INTERVENTION(S)

Phospholipase C zeta (PLCzeta), the putative mammalian egg activation factor, was fused to enhanced yellow fluorescent protein (EYFP), and in vivo gene transfer by electroporation was used to introduce this transgene (PLCzeta-EYFP) into mouse testis. Transgene expression in testis and sperm were analyzed at 20 and 40 days after electroporation.

MAIN OUTCOME MEASURE(S)

Transgene expression in testis and epididymal sperm was analyzed by fluorescence microscopy and an excitation light source suitable for EYFP.

RESULT(S)

Phospholipase C zeta-EYFP was successfully expressed in epididymal sperm when analyzed 40 days after gene transfer and was localized to the head and midpiece regions.

CONCLUSION(S)

Our results provide the first demonstration that in vivo gene transfer can be used to study the localization of proteins in mature sperm and that this represents a powerful new technique for studying male infertility and gene function in sperm.

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.

Mammalian egg activation: from Ca2+ spiking to cell cycle progression

Mammalian eggs arrest at metaphase of the second meiotic division (MetII). Sperm break this arrest by inducing a series of Ca2+spikes that last for several hours. During this time cell cycle resumption is induced, sister chromatids undergo anaphase and the second polar body is extruded. This is followed by decondensation of the chromatin and the formation of pronuclei. Ca2+spiking is both the necessary and solely sufficient sperm signal to induce full egg activation. How MetII arrest is established, how the Ca2+spiking is induced and how the signal is transduced into cell cycle resumption are the topics of this review. Although the roles of most components of the signal transduction pathway remain to be fully investigated, here I present a model in which a sperm-specific phospholipase C (PLCζ) generates Ca2+spikes to activate calmodulin-dependent protein kinase II and so switch on the Anaphase-Promoting Complex/Cyclosome (APC/C). APC/C activation leads to securin and cyclin B1 degradation and in so doing allows sister chromatids to be segregated and to decondense.

Calmodulin-dependent protein kinase II, and not protein kinase C, is sufficient for triggering cell-cycle resumption in mammalian eggs

Mouse eggs arrest at metaphase II following ovulation and are only triggered to complete meiosis when fertilized. Sperm break the cell-cycle arrest by a long-lasting series of Ca2+ spikes that lead to an activation of the anaphase-promoting complex/cyclosome. The signal transduction pathway is not fully resolved but both protein kinase C (PKC) and calmodulin-dependent protein kinase II (CamKII) activities increase at fertilization and previous pharmacological studies have implicated both in cell-cycle resumption. We have used a combination of pharmacological inhibitors and constitutively active cRNA constructs of PKCα and CamKIIα microinjected into mouse eggs to show that it is CamKII and not PKC that is the sufficient trigger for cell-cycle resumption from metaphase II arrest.

Constitutively active PKC constructs had no effect on the resumption of meiosis but caused an immediate and persistent elevation in intracellular Ca2+ when store-operated Ca2+ entry was stimulated. With respect to resumption of meiosis, the effects of constitutively active CamKII on eggs were the same as sperm. Eggs underwent second polar body extrusion and pronucleus formation with normal timings; while both securin and cyclin B1 destruction, visualised by coupling to fluorescent protein tags, were complete by the time of polar body extrusion. Induction of a spindle checkpoint by overexpression of Mad2 or by spindle poisons blocked CamKII-induced resumption of meiosis, but the Ca2+ chelator BAPTA did not. Furthermore direct measurement of Ca2+ levels showed that CamKII did not induce exit from metaphase II arrest by raising Ca2+. Therefore, we conclude that PKCs may play an important role in maintaining Ca2+ spiking at fertilization by promoting store-operated Ca2+ entry, while CamKII transduces cell-cycle resumption, and lies downstream of sperm-induced Ca2+ release but upstream of a spindle checkpoint. These data, combined with the knowledge that CamKII activity increase at fertilization, suggest that mouse eggs undergo cell-cycle resumption through stimulation of CamKII.

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.

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.