Remodeling of the specialized intermediate filament network in mammalian eggs and embryos during development: regulation by protein kinase C and protein kinase M

Role for PADI6 in securing the mRNA-MSY2 complex to the oocyte cytoplasmic lattices

The oocyte cytoplasmic lattices (CPLs) have long been predicted to function as a storage form for the maternal contribution of ribosomes to the early embryo. Our previous studies have demonstrated that ribosomal component S6 is stored in the oocyte CPLs and peptidylarginine deiminase 6 (PADI6) is critical for CPLs formation. Additionally, we found that depletion of PADI6 reduced de novo protein synthesis prior to the maternal-to-embryonic transition, therefore causing embryos to arrest at the 2-cell stage. Here, we present evidence further supporting the association of ribosomes with the CPLs by demonstrating that rRNAs are dramatically decreased in Padi6 KO oocytes. We also show that the abundance and localization of mRNAs is affected upon PADI6 depletion, suggesting that mRNAs are very possibly associated with CPLs. Consistent with this observation, the amount of the major RNA binding protein, MSY2, that is associated with the insoluble fraction of the oocytes after Triton X-100 extraction is also markedly decreased in the Padi6 KO oocytes. Furthermore, treatment of the oocytes with RNase A followed by Triton X-100 extraction severely impairs the localization of PADI6 and MSY2 in oocytes. These results indicate that mRNAs, possibly in a complex with MSY2 and PADI6, are bound in the CPLs and may play a role in securing the mRNA-MSY2 complex to the CPLs.

miRNA-17 members that target Bmpr2 influence signaling mechanisms important for embryonic stem cell differentiation in vitro and gastrulation in embryos

Body axes and germ layers evolve at gastrulation, and in mammals are driven by many genes; however, what orchestrates the genetic pathways during gastrulation remains elusive. Previously, we presented evidence that microRNA-17 (miRNA-17) family members, miR-17-5p, miR-20a, miR-93, and miR-106a were differentially expressed in mouse embryos and functioned to control differentiation of the stem cell population. Here, we identify function(s) that these miRNAs have during gastrulation. Fluorescent in situ hybridization miRNA probes reveal that these miRNAs are localized at the mid/posterior primitive streak (ps) in distinct populations of primitive ectoderm, mesendoderm, and mesoderm. Seven different miRNA prediction algorithms are identified in silico bone morphogenic protein receptor 2 (Bmpr2) as a target of these miRNAs. Bmpr2 is a member of the TGFβ pathway and invokes stage-specific changes during gastrulation. Recently, Bmpr2 was shown regulating cytoskeletal dynamics, cell movement, and invasion. Our previous and current data led to a hypothesis by which members of the miR-17 family influence gastrulation by suppressing Bmpr2 expression at the primitive streak. This suppression influences fate decisions of cells by affecting genes downstream of BMPR2 as well as mesoderm invasion through regulation of actin dynamics.

The roles of Ca2+, downstream protein kinases, and oscillatory signaling in regulating fertilization and the activation of development

Reviews in Developmental Biology have covered the pathways that generate the all-important intracellular calcium (Ca(2+)) signal at fertilization [Miyazaki, S., Shirakawa, H., Nakada, K., Honda, Y., 1993a. Essential role of the inositol 1,4,5-trisphosphate receptor/Ca(2+) release channel in Ca(2+) waves and Ca(2+) oscillations at fertilization of mammalian eggs. Dev. Biol. 158, 62-78; Runft, L., Jaffe, L., Mehlmann, L., 2002. Egg activation at fertilization: where it all begins. Dev. Biol. 245, 237-254] and the different temporal responses of Ca(2+) in many organisms [Stricker, S., 1999. Comparative biology of calcium signaling during fertilization and egg activation in animals. Dev. Biol. 211, 157-176]. Those reviews raise the importance of identifying how Ca(2+) causes the events of egg activation (EEA) and to what extent these temporal Ca(2+) responses encode developmental information. This review covers recent studies that have analyzed how these Ca(2+) signals are interpreted by specific proteins, and how these proteins regulate various EEA responsible for the onset of development. Many of these proteins are protein kinases (CaMKII, PKC, MPF, MAPK, MLCK) whose activity is directly or indirectly regulated by Ca(2+), and whose amount increases during late oocyte maturation. We cover biochemical progress in defining the signaling pathways between Ca(2+) and the EEA, as well as discuss how oscillatory or multiple Ca(2+) signals are likely to have specific advantages biochemically and/or developmentally. These emerging concepts are put into historical context, emphasizing that key contributions have come from many organisms. The intricate interdependence of Ca(2+), Ca(2+)-dependent proteins, and the EEA raise many new questions for future investigations that will provide insight into the extent to which fertilization-associated signaling has long-range implications for development. In addition, answers to these questions should be beneficial to establishing parameters of egg quality for human and animal IVF, as well as improving egg activation protocols for somatic cell nuclear transfer to generate stem cells and save endangered species.

Dynamics and unexpected localization of the plakin binding protein, kazrin, in mouse eggs and early embryos

The cell uses the cytoskeleton in virtually every aspect of cell survival and function. One primary function of the cytoskeleton is to connect to and stabilize intercellular junctions. To accomplish this task, microtubules, actin filaments, and intermediate filaments utilize cytolinker proteins, which physically bind the cytoskeletal filament to the core proteins of the adhesion junction. The plakin family of linker proteins have been in the spotlight recently as critical components for embryo survival and, when mutated, the cause of diseases such as muscular dystrophy and cardiomyopathies. Here, we reveal the dynamics of a recently discovered plakin binding protein, kazrin (kaz), during early mouse development. Kaz was originally found in adult tissues, primarily epidermis, linking periplakin to the plasma membrane and colocalizing with desmoplakin in desmosomes. Using reverse transcriptase-polymerase chain reaction, Western blots, and confocal microscopy, we found kaz in unfertilized eggs associated with the spindle apparatus and cytoskeletal sheets. As quickly as 5 min after egg activation, kaz relocates to a diffuse peri-spindle position, followed 20-30 min later by clear localization to the presumptive cytokinetic ring. Before the blastocyst stage of development, kaz associates with the nuclear matrix in a cell cycle-dependent manner, and also associates with the cytoplasmic actin cytoskeleton. After blastocyst formation, kaz localization and potential function(s) become highly complex as it is found associating with cell-cell junctions, the cytoskeleton, and nucleus. Postimplantation stages of development reveal that kaz retains a multifunctional, tissue-specific role as it is detected at diverse locations in various embryonic tissue types.

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.

ePAD, an oocyte and early embryo-abundant peptidylarginine deiminase-like protein that localizes to egg cytoplasmic sheets

Selected for its high relative abundance, a protein spot of MW approximately 75 kDa, pI 5.5 was cored from a Coomassie-stained two-dimensional gel of proteins from 2850 zona-free metaphase II mouse eggs and analyzed by tandem mass spectrometry (TMS), and novel microsequences were identified that indicated a previously uncharacterized egg protein. A 2.4-kb cDNA was then amplified from a mouse ovarian adapter-ligated cDNA library by RACE-PCR, and a unique 2043-bp open reading frame was defined encoding a 681-amino-acid protein. Comparison of the deduced amino acid sequence with the nonredundant database demonstrated that the protein was approximately 40% identical to the calcium-dependent peptidylarginine deiminase (PAD) enzyme family. Northern blotting, RT-PCR, and in situ hybridization analyses indicated that the protein was abundantly expressed in the ovary, weakly expressed in the testis, and absent from other tissues. Based on the homology with PADs and its oocyte-abundant expression pattern, the protein was designated ePAD, for egg and embryo-abundant peptidylarginine deiminase-like protein. Anti-recombinant ePAD monospecific antibodies localized the molecule to the cytoplasm of oocytes in primordial, primary, secondary, and Graafian follicles in ovarian sections, while no other ovarian cell type was stained. ePAD was also expressed in the immature oocyte, mature egg, and through the blastocyst stage of embryonic development, where expression levels began to decrease. Immunoelectron microscopy localized ePAD to egg cytoplasmic sheets, a unique keratin-containing intermediate filament structure found only in mammalian eggs and in early embryos, and known to undergo reorganization at critical stages of development. Previous reports that PAD-mediated deimination of epithelial cell keratin results in cytoskeletal remodeling suggest a possible role for ePAD in cytoskeletal reorganization in the egg and early embryo.

Paternal mRNA and protein synthesis coincides with male chromatin decondensation in maize zygotes

Decondensation of the male genome after fertilization is a prerequisite for replication and transcription. Cytological analysis has revealed decondensation of the male chromatin to commence immediately after karyogamy and progress rapidly, pointing to an early start of transcription. To investigate early transcription from the paternal genome in maize zygotes, we generated transgenic plants containing green fluorescent protein (GFP) under control of the 35S promoter. Single transgenic sperm cells from these plants were used to fertilize isolated wild-type egg cells in vitro. These sperm cells did not contain gfp transcripts. Appearance of gfp mRNA, 4 h after fertilization, was coincident with decondensation of the male chromatin, and clearly demonstrates early accessibility to the transcriptional machinery of at least a part of the male genome. Translational activity in early zygotes was evident 6 h after fertilization, as demonstrated by measurable levels of GFP fluorescence signal. Using a similar strategy, we also demonstrated activity of the paternal genome early in endosperm development. These findings may exclude any global mechanism of silencing the entire paternal genome over this period, and make an almost immediate paternal contribution to zygote and early endosperm development conceivable. These data are also considered in the perspective of current views of genome activation in the zygotes and young embryos of animals.

Rescuing desmoplakin function in extra-embryonic ectoderm reveals the importance of this protein in embryonic heart, neuroepithelium, skin and vasculature

Desmosomes mediate intercellular adhesion through desmosomal cadherins, which interface with plakoglobin (PG) and desmoplakin (DP) to associate with the intermediate filament (IF) cytoskeleton. Desmosomes first assemble in the E3.5 mouse trophectoderm, concomitant with establishment of epithelial polarity and appearance of a blastocoel cavity. Increasing in size and number, desmosomes continue their prominence in extra-embryonic tissues, but as development proceeds, they also become abundant in a number of embryonic tissues, including heart muscle, epidermis and neuroepithelium. Previously, we explored the functional importance of desmosomes by ablating the Dsp gene. Homozygous Dsp mutant embryos progressed through implantation, but did not survive beyond E6.5, owing to a loss or instability of desmosomes and tissue integrity. We have now rescued the extra-embryonic tissues by aggregation of tetraploid (wild-type) and diploid (Dsp mutant) morulae. These animals survive several days longer, but die shortly after gastrulation, with major defects in the heart muscle, neuroepithelium and skin epithelium, all of which possess desmosomes, as well as the microvasculature, which does not. Interestingly, although wild-type endothelial cells of capillaries do not form desmosomes, they possess unusual intercellular junctions composed of DP, PG and VE-cadherin. The severity in phenotype and the breadth of defects in the Dsp mutant embryo is greater than PG mutant embryos, substantiating redundancy between PG and other armadillo proteins (e.g. beta-catenin). The timing of lethality is similar to that of the VE-cadherin null embryo, suggesting that a participating cause of death may be a defect in vasculature, not reported for PG null embryos.

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.

Novel mechanism of protein kinase C inhibition involving the pseudosubstrate region by secalonic acid D in vitro

Evidence from studies in mice suggests a mechanistic role for the inhibition of conventional isoforms of protein kinase C (cPKC) in the development of cleft palate (CP) in the offspring of female mice treated with the mycotoxin, secalonic acid D (SAD). These experiments were aimed at assessing whether SAD inhibits commercially available pure cPKC (PKCalpha, -beta, -gamma) and at identifying the mechanism of such an inhibition in vitro. Secalonic acid D inhibited the three isozymes similarly (IC50 of 5 to 6.2 microM by direct extrapolation and 2.7 to 4 microM by logarithmic regression). The loss of inhibitory effect of SAD upon removal of the regulatory domain of PKCbetaII, the most predominant cPKC in the palate, suggested that the inhibition was mediated by the regulatory subunit. Kinetic analysis suggested a lack of competitive interaction for SAD with the binding sites for Ca(2+) and phosphatidyl serine (PS). Antibody directed against residues 19-32 of the pseudosubstrate region of PKCbetaII, however, competitively reversed the inhibition of PKCbetaII by SAD, suggesting that the pseudosubstrate is the site of interaction of SAD. Further, SAD inhibited the cleavage of the pseudosubstrate from PKCbetaII by the endoproteinase Arg-C. The fact that the activity of Arg-C itself was not inhibited by SAD suggests that SAD interferes with the preceding step involving the cofactor-induced release of the pseudosubstrate from the active site of PKCbetaII, a novel mechanism.

Activation of protein kinase C after fertilization is required for remodeling the mouse egg into the zygote

Fertilization of the mammalian egg initiates numerous biochemical and structural changes which remodel the egg into a single-celled zygote. To date, the most extensively studied phenomenon of fertilization in virtually all species has been the relationship between sperm penetration and the induction of the initial rise in intracellular-free calcium ([Ca2+]i) concentration within the egg. In contrast, relatively few studies have focused on the biochemical events following this rise in calcium, and even fewer studies have directly linked the biochemical events to the structural changes which must ensue for proper development of the embryo. In this study, we exploited recently developed technologies to investigate the action of protein kinase C (PKC), a presumed downstream transducer of the initial rise in [Ca2+]i, during fertilization and artificial activation with calcium ionophore or phorbol 12-myristate 13-acetate (PMA). The newly developed myristoylated PKC pseudosubstrate (myrPKC psi) was used to specifically inhibit PKC, thereby averting the trauma of injecting the egg with nonmyristoylated PKC psi. Following fertilization, eggs which were pretreated with myr-PKC psi were not capable of forming a second polar body and pronuclear formation was significantly inhibited. Spatial and temporal localization of PKC using confocal microscopy to visualize the PKC reporter dye, Rim-1, demonstrated localization of PKC to the lateral aspects of the forming second polar body after fertilization, or after artificial activation with calcium ionophore or PMA. In vivo biochemical analysis of eggs which were fertilized or artificially activated demonstrated that PKC activity rose at the same time (40 min) as the second polar body formed and then subsided over the next 5 hr post activation. From these data, we conclude that PKC plays an integral role in directing the transformation from egg to embryo.

PKC--a pivotal regulator of early development

Oocytes, eggs and blastomeres of the embryo are special cells that undergo rapid changes in structure and function at developmental transitions. These changes are frequently regulated by cytoplasmic signaling events, particularly at the developmental transition of fertilization, because the genome is largely inactivated at this time. Protein kinase C (PKC) is a signaling agent that acts after the sperm-induced rise in calcium and has a central role in the remodeling of the structure of the egg into the zygote in many species. PKC also acts during other developmental transitions. This kinase serves as a chronometer, which can choreograph the cell's remodeling events in both space and time. Several technical advancements discussed in this review have permitted a better understanding of the actions of PKC.