A Freeze-Sectioning Method for Preparation of the Detergent-Resistant Cytoskeleton Identifies Stage-Specific Cytoskeleal Proteins and Associated mRNA in Xenopus Oocytes and Embryos. (Cytoskeleton/amphibian/mRNA)

The role of protein kinase C in reorganization of the cortical cytoskeleton during the transition from oocyte to fertilization-competent egg

Fertilization-competent amphibian eggs (metaphase II) are programmed to undergo an actin-myosin based contraction of the cortical cytoplasm (i.e., cortical contraction) in response to an elevation of intracellular-free calcium which accompanies fertilization. This ability to undergo cortical contraction is acquired within a few hours after the meiotically-arrested oocyte is triggered to resume meiosis by exposure to progesterone. This report examines the timing of changes in the contractile potential of the cortical cytoplasm as the oocyte becomes the egg, and in addition, the signal transduction events which induce these changes. We use the bisected oocyte system developed by Christensen et al. ('84; Nature 310: 150-151) to assess the changes in cortical potential during the meiotic resumption. Immediately after progesterone treatment (less than 5% of the way through the meiotic resumption) the cortex acquires the ability to form a contractile ring, an ability which gradually disappears during the meiotic resumption. Eighty percent of the way through the meiotic resumption the cortex of the hemisphere rapidly acquires the ability to undergo cortical contraction. In contrast, when bisected in a medium containing protein kinase C (PKC) agonists, the cortex of the hemisphere undergoes cortical contraction much earlier (i.e., 50% through the meiotic resumption). In addition, treatment of oocytes with PKC agonists alone can mimic the complete spectrum of changes in cortical potential induced by progesterone, suggesting that PKC has a role in reorganization of the cortical cytoskeleton which occurs as a normal response to progesterone. In support of this, antagonists of PKC block the progesterone-induced reorganization of the cortical cytoskeleton.

Role of the cytoskeleton during early development

Oocytes, eggs, and embryos from a diverse array of species have evolved cytoskeletal specializations which allow them to meet the needs of early embryogenesis. While each species studied possesses one or more specializations which are unique, several cytoskeletal features are widely conserved across different animal phyla. These features include highly-developed cortical cytoskeletal domains associated with developmental information, microtubule-mediated pronuclear transport, and rapid intracellular signal-regulated control of cytoskeletal organization.

Analysis of inducible contractile rings suggests a role for protein kinase C in embryonic cytokinesis and wound healing

A semi-in vitro system derived from Xenopus oocytes which allows induction of contractile ring (CR) formation and closure is described and exploited to elucidate regulatory and structural features of cytokinesis. The inducible CRs (ICRs) are composed of actin filaments and closure is actin filament-dependent as is cytokinesis in vivo. ICR closure in this system is calcium-dependent and pH-sensitive, as is cytokinesis in permeabilized cells (Cande: Journal of Cell Biology 87:326, 1980). Closure of ICRs proceeds at a rate and with a kinetic pattern similar to embryonic cytokinesis. Collectively, these data demonstrate that this system is a faithful mimic of cytokinesis in vivo. ICR formation and closure is protein kinase C (PKC)-dependent and neomycin-sensitive, indicating that the PKC branch of the polyphosphoinositide pathway regulates formation of the actomyosin ring which is the effector of cytokinesis. Kinetic measurements show that the rate of ICR closure reaches a peak of 4-8 microns/sec. Since the maximum measured velocity of actin filament translocation by vertebrate, non-muscle myosins is 0.04 micron/sec, the later observations support a model in which the CR is segmented, containing multiple sites where filaments overlap in a "sliding filament" fashion. Because the rate decreases after reaching a peak, the results also suggest that the number of overlap sites decrease with time.

Transformation of the amphibian oocyte into the egg: structural and biochemical events

Amphibian oocytes, arrested in prophase I, are stimulated to progress to metaphase II by progesterone. This process is referred to as meiotic maturation and transforms the oocyte, which cannot support the early events of embryogenesis, into the egg, which can. Meiotic maturation entails global reorganization of cell ultrastructure: In the cell cortex, the plasma membrane flattens and the cortical granules undergo redistribution. In the cell periphery, the annulate lamellae disassemble and the mitochondria become dispersed. In the cell interior, the germinal vesicle becomes disassembled and the meiotic spindles form. Marked changes in the cytoskeleton and mRNA distribution also occur throughout the cell. All of these events are temporally correlated with intracellular signalling events: Fluctuations in cAMP levels, changes in pH, phosphorylation and dephosphorylation, and ion flux changes. Evidence suggests that specific intracellular signals are responsible for specific reorganizations of ultrastructure and mRNA distribution.

The cortical cytoskeleton and its role in sperm penetration of the mammalian egg

In this study isolated cortical regions of both penetrated and nonpenetrated Syrian hamster eggs were examined in whole mounts and platinum replicas of detergent-extracted cortical patches. Two types of cytoskeletal organization were observed in the egg cortex: Loose networks (LN regions) with integrated localized dense networks (LDN regions). Decoration with heavy meromyosin and labeling with antiactin/protein G gold both indicate that the cortical cytoskeleton consists mainly of a LN of actin microfilaments and several types of nonactin filaments, whereas LDN regions dispersed within the LN were comprised of nonactin filaments. Cortical patches and replicas of eggs incubated with sperm for 10-15 min provide evidence that cortical microfilaments may be intimately associated with penetrating spermatozoa. The results of this investigation provide the first high resolution view of the cortical cytoskeletal domain of a mammalian egg and suggest that actin microfilaments might play a role in sperm penetration of the egg cortex.

Functions of maternal mRNA in early development

In this review, the types of mRNAs found in oocytes and eggs of several animal species, particularly Drosophila, marine invertebrates, frogs, and mice, are described. The roles that proteins derived from these mRNAs play in early development are discussed, and connections between maternally inherited information and embryonic pattern are sought. Comparisons between genetically identified maternally expressed genes in Drosophila and maternal mRNAs biochemically characterized in other species are made when possible. Regulation of the meiotic and early embryonic cell cycles is reviewed, and translational control of maternal mRNA following maturation and/or fertilization is discussed with regard to specific mRNAs.