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

Dithiothreitol prevents the spontaneous release of cortical granules in in vitro aged mouse oocytes by protecting regulatory proteins of cortical granules exocytosis and thickening the cortical actin cytoskeleton

In assisted fertility protocols, in vitro culture conditions mimic physiological conditions to preserve gametes in the best conditions. After collection, oocytes are maintained in a culture medium inside the incubator until in vitro fertilization (IVF) is performed. This time outside natural and physiological conditions exposes oocytes to an oxidative stress that renders in vitro aging. It has been described that in vitro aging produces a spontaneous cortical granule (CG) release decreasing the fertilization rate of oocytes. Nevertheless, this undesirable phenomenon has not been investigated, let alone prevented. In this work, we characterized the spontaneous CG secretion in in vitro aged oocytes. Using immunofluorescence indirect, quantification, and functional assays, we showed that the expression of regulatory proteins of CG exocytosis was affected. Our results demonstrated that in vitro oocyte aging by 4 and 8 h altered the expression and localization of alpha-SNAP and reduced the expression of NSF and Complexin. These alterations were prevented by supplementing culture medium with dithiothreitol (DTT), which in addition to having a protective effect on those proteins, also had an unexpected effect on the actin cytoskeleton. Indeed, DTT addition thickened the cortical layer of fibrillar actin. Both DTT effects, together, prevented the spontaneous secretion of CG and recovered the IVF rate in in vitro aged oocytes. We propose the use of DTT in culture media to avoid the spontaneous CG secretion and to improve the success rate of IVF protocols in in vitro aged oocytes.

Dehydroleucodine and xanthatin, two natural anti-inflammatory lactones, inhibit mast cell degranulation by affecting the actin cytoskeleton

Actin remodeling is a critical regulator of mast cell secretion. In previous work, we have shown that dehydroleucodine and xanthatin, two natural α,β-unsaturated lactones, exhibit anti-inflammatory and mast cell stabilizing properties. Based on this background, this study aimed to determine whether the mast cell stabilizing action of these lactones is associated with changes in the actin cytoskeleton. Rat peritoneal mast cells were preincubated in the presence of dehydroleucodine or xanthatin before incubation with compound 48/80. Comparative studies with sodium cromoglycate and latrunculin B were also made. After treatments, different assays were performed on mast cell samples: β-hexosaminidase release, cell viability studies, quantification of mast cells and their state of degranulation by light microscopy, transmission electron microscopy, and actin staining for microscopy observation. Results showed that dehydroleucodine and xanthatin inhibited mast cell degranulation, evidenced by the inhibition of β-hexosaminidase release and decreased degranulated mast cell percentage. At the same time, both lactones altered the F-actin cytoskeleton in mast cells resulting, similarly to Latrunculin B, in a higher concentration of nuclear F-actin when activated by compound 48/80. For the first time, this study describes the biological properties of dehydroleucodine and xanthatin concerning to the rearrangement of actin filaments during stimulated exocytosis in mast cells. These data have important implications for developing new anti-inflammatory and mast cell stabilizing drugs and for designing new small molecules that may interact with the actin cytoskeleton.

Pathophysiological roles of myristoylated alanine-rich C-kinase substrate (MARCKS) in hematological malignancies

The myristoylated alanine-rich C-kinase substrate (MARCKS) protein has been at the crossroads of multiple signaling pathways that govern several critical operations in normal and malignant cellular physiology. Functioning as a target of protein kinase C, MARCKS shuttles between the phosphorylated cytosolic form and the unphosphorylated plasma membrane-bound states whilst regulating several molecular partners including, but not limited to calmodulin, actin, phosphatidylinositol-4,5-bisphosphate, and phosphoinositide-3-kinase. As a result of these interactions, MARCKS directly or indirectly modulates a host of cellular functions, primarily including cytoskeletal reorganization, membrane trafficking, cell secretion, inflammatory response, cell migration, and mitosis. Recent evidence indicates that dysregulated expression of MARCKS is associated with the development and progression of hematological cancers. While it is understood that MARCKS impacts the overall carcinogenesis as well as plays a part in determining the disease outcome in blood cancers, we are still at an early stage of interpreting the pathophysiological roles of MARCKS in neoplastic disease. The situation is further complicated by contradictory reports regarding the role of phosphorylated versus an unphosphorylated form of MARCKS as an oncogene versus tumor suppressor in blood cancers. In this review, we will investigate the current body of knowledge and evolving concepts of the physical properties, molecular network, functional attributes, and the likely pathogenic roles of MARCKS in hematological malignancies. Key emphasis will also be laid upon understanding the novel mechanisms by which MARCKS determines the overall disease prognosis by playing a vital role in the induction of therapeutic resistance. Additionally, we will highlight the importance of MARCKS as a valuable therapeutic target in blood cancers and will discuss the potential of existing strategies available to tackle MARCKS-driven blood cancers.

MARCKS and Lung Disease

MARCKS (myristoylated alanine-rich C kinase substrate) is a prominent PKC substrate expressed in all eukaryotic cells. It is known to bind to and cross-link actin filaments, to serve as a bridge between Ca²⁺/calmodulin and PKC signaling, and to sequester the signaling molecule phosphatidylinositol 4,5-bisphosphate in the plasma membrane. Since the mid-1980s, this evolutionarily conserved and ubiquitously expressed protein has been associated with regulating cellular events that require dynamic actin reorganization, including cellular adhesion, migration, and exocytosis. More recently, translational studies have implicated MARCKS in the pathophysiology of a number of airway diseases, including chronic obstructive pulmonary disease, asthma, lung cancer, and acute lung injury/acute respiratory distress syndrome. This article summarizes the structure and cellular function of MARCKS (also including MARCKS family proteins and MARCKSL1 [MARCKS-like protein 1]). Evidence for MARCKS's role in several lung diseases is discussed, as are the technological innovations that took MARCKS-targeting strategies from theoretical to therapeutic. Descriptions and updates derived from ongoing clinical trials that are investigating inhalation of a MARCKS-targeting peptide as therapy for patients with chronic bronchitis, lung cancer, and ARDS are provided.

Spindle tubulin and MTOC asymmetries may explain meiotic drive in oocytes

In the first meiotic division (MI) of oocytes, the cortically positioned spindle causes bivalent segregation in which only the centre-facing homologue pairs are retained. 'Selfish' chromosomes are known to exist, which bias their spindle orientation and hence retention in the egg, a process known as 'meiotic drive'. Here we report on this phenomenon in oocytes from F₁ hybrid mice, where parental strain differences in centromere size allows distinction of the two homologue pairs of a bivalent. Bivalents with centromere and kinetochore asymmetry show meiotic drive by rotating during prometaphase, in a process dependent on aurora kinase activity. Cortically positioned homologue pairs appear to be under greater stretch than their centre-facing partners. Additionally the cortex spindle-half contain a greater density of tubulin and microtubule organising centres. A model is presented in which meiotic drive is explained by the impact of microtubule force asymmetry on chromosomes with different sized centromeres and kinetochores.

Phosphoproteome of Toxoplasma gondii Infected Host Cells Reveals Specific Cellular Processes Predominating in Different Phases of Infection

The invasion of Toxoplasma gondii tachyzoites into the host cell results in extensive host cell signaling activation/deactivation that is usually regulated by the phosphorylation/dephosphorylation. To elucidate how T. gondii regulates host cell signal transduction, the comparative phosphoproteome of stable isotope labeling with amino acids in cell culture-labeled human foreskin fibroblast cells was analyzed. The cells were grouped (Light [L], Medium [M], and Heavy [H] groups) based on the labeling isotope weight and were infected with T. gondii for different lengths of time (L: 0 hour; M: 2 hours; and H: 6 hours). A total of 892 phosphoproteins were identified with 1,872 phosphopeptides and 1,619 phosphorylation sites. The M versus L comparison revealed 694 significantly regulated phosphopeptides (436 upregulated and 258 downregulated). The H versus L comparison revealed 592 significantly regulated phosphopeptides (146 upregulated and 446 downregulated). The H versus M comparison revealed 794 significantly regulated phosphopeptides (149 upregulated and 645 downregulated). At 2 and 6 hours post-T. gondii infection, the most predominant host cell reactions were cell cycle regulation and cytoskeletal reorganization, which might be required for the efficient invasion and multiplication of T. gondii. Similar biological process profiles but different molecular function categories of host cells infected with T. gondii for 2 and 6 hours, which suggested that the host cell processes were not affected significantly by T. gondii infection but emphasized some differences in specific cellular processes at this two time points. Western blotting verification of some significantly regulated phosphoprotein phosphorylation sites was consistent with the mass spectra data. This study provided new insights into and further understanding of pathogen-host interactions from the host cell perspective.

Functional Diversification of the Four MARCKS Family Members in Zebrafish Neural Development

Myristoylated alanin-rich C-kinase substrate (MARCKS) and MARCKS-like 1, each encoded by a different gene, comprise a very small family of actin-modulating proteins with essential roles in mammalian neural development. We show here that four genes (two marcks and two marcksl1) are present in teleosts including zebrafish, while ancient actinopterigians, sarcopterigian fishes, and chondrichtyans only have two. No marcks genes were found in agnaths or invertebrates. All four zebrafish genes are expressed during development, and we show here how their early knockdown causes defects in neural development, with some phenotypical differences. Knockdown of marcksa generated embryos with smaller brain and eyes, while marcksb caused different morphogenetic defects, such as larger hindbrain ventricle and folded retina. marcksl1a and marcksl1b morpholinos also caused smaller eyes and brain, although marcksl1a alone generated larger brain ventricles. At 24 hpf, marcksb caused a wider angle of the hindbrain walls, while marcksl1a showed a "T-shaped" neural tube and alterations in neuroepithelium organization. The double knockdown surprisingly produced new features, which included an increased neuroepithelial disorganization and partial neural tube duplications evident at 48 hpf, suggesting defects in convergent extension. This disorganization was also evident in the retina, although retinal ganglion cells were still able to differentiate. marcksl1b morphants presented a unique retinal phenotype characterized by the occurrence of sporadic ectopic neuronal differentiation. Although only marcksl1a morphant had a clear "ciliary phenotype," all presented significantly shorter cilia. Altogether, our data show that all marcks genes have functions in zebrafish neural development, with some differences that suggest the onset of protein diversification.

Rab3A, a possible marker of cortical granules, participates in cortical granule exocytosis in mouse eggs

Fusion of cortical granules with the oocyte plasma membrane is the most significant event to prevent polyspermy. This particular exocytosis, also known as cortical reaction, is regulated by calcium and its molecular mechanism is still not known. Rab3A, a member of the small GTP-binding protein superfamily, has been implicated in calcium-dependent exocytosis and is not yet clear whether Rab3A participates in cortical granules exocytosis. Here, we examine the involvement of Rab3A in the physiology of cortical granules, particularly, in their distribution during oocyte maturation and activation, and their participation in membrane fusion during cortical granule exocytosis. Immunofluorescence and Western blot analysis showed that Rab3A and cortical granules have a similar migration pattern during oocyte maturation, and that Rab3A is no longer detected after cortical granule exocytosis. These results suggested that Rab3A might be a marker of cortical granules. Overexpression of EGFP-Rab3A colocalized with cortical granules with a Pearson correlation coefficient of +0.967, indicating that Rab3A and cortical granules have almost a perfect colocalization in the egg cortical region. Using a functional assay, we demonstrated that microinjection of recombinant, prenylated and active GST-Rab3A triggered cortical granule exocytosis, indicating that Rab3A has an active role in this secretory pathway. To confirm this active role, we inhibited the function of endogenous Rab3A by microinjecting a polyclonal antibody raised against Rab3A prior to parthenogenetic activation. Our results showed that Rab3A antibody microinjection abolished cortical granule exocytosis in parthenogenetically activated oocytes. Altogether, our findings confirm that Rab3A might function as a marker of cortical granules and participates in cortical granule exocytosis in mouse eggs.

The Effector Domain of MARCKS Is a Nuclear Localization Signal that Regulates Cellular PIP2 Levels and Nuclear PIP2 Localization

Translocation to the nucleus of diacylglycerol kinase (DGK)– ζ is dependent on a sequence homologous to the effector domain of Myristoylated Alanine Rich C-Kinase Substrate (MARCKS). These data would suggest that MARCKS could also localize to the nucleus. A single report demonstrated immunofluorescence staining of MARCKS in the nucleus; however, further experimental evidence confirming the specific domain responsible for this localization has not been reported. Here, we report that MARCKS is present in the nucleus in GBM cell lines. We then over-expressed wild-type MARCKS (WT) and MARCKS with the effector domain deleted (ΔED), both tagged with V5-epitope in a GBM cell line with low endogenous MARCKS expression (U87). We found that MARCKS-WT localized to the nucleus, while the MARCKS construct without the effector domain remained in the cytoplasm. We also found that over-expression of MARCKS-WT resulted in a significant increase in total cellular phosphatidyl-inositol (4,5) bisphosphate (PIP₂) levels, consistent with prior evidence that MARCKS can regulate PIP₂ levels. We also found increased staining for PIP₂ in the nucleus with MARCKS-WT over-expression compared to MARCKS ΔED by immunofluorescence. Interestingly, we observed MARCKS and PIP₂ co-localization in the nucleus. Lastly, we found changes in gene expression when MARCKS was not present in the nucleus (MARCKS ΔED). These data indicate that the MARCKS effector domain can function as a nuclear localization signal and that this sequence is critical for the ability of MARCKS to regulate PIP₂ levels, nuclear localization, and gene expression. These data suggests a novel role for MARCKS in regulating nuclear functions such as gene expression.

X MARCKS the spot: myristoylated alanine-rich C kinase substrate in neuronal function and disease

Intracellular protein-protein interactions are dynamic events requiring tightly regulated spatial and temporal checkpoints. But how are these spatial and temporal cues integrated to produce highly specific molecular response patterns? A helpful analogy to this process is that of a cellular map, one based on the fleeting localization and activity of various coordinating proteins that direct a wide array of interactions between key molecules. One such protein, myristoylated alanine-rich C-kinase substrate (MARCKS) has recently emerged as an important component of this cellular map, governing a wide variety of protein interactions in every cell type within the brain. In addition to its well-documented interactions with the actin cytoskeleton, MARCKS has been found to interact with a number of other proteins involved in processes ranging from intracellular signaling to process outgrowth. Here, we will explore these diverse interactions and their role in an array of brain-specific functions that have important implications for many neurological conditions.

Molecular mechanisms of asymmetric division in oocytes

In contrast to symmetric division in mitosis, mammalian oocyte maturation is characterized by asymmetric cell division that produces a large egg and a small polar body. The asymmetry results from oocyte polarization, which includes spindle positioning, migration, and cortical reorganization, and this process is critical for fertilization and the retention of maternal components for early embryo development. Although actin dynamics are involved in this process, the molecular mechanism underlying this remained unclear until the use of confocal microscopy and live cell imaging became widespread in recent years. Information obtained through a PubMed database search of all articles published in English between 2000 and 2012 that included the phrases "oocyte, actin, spindle migration," "oocyte, actin, polar body," or "oocyte, actin, asymmetric division" was reviewed. The actin nucleation factor actin-related protein 2/3 complex and its nucleation-promoting factors, formins and Spire, and regulators such as small GTPases, partitioning-defective/protein kinase C, Fyn, microRNAs, cis-Golgi apparatus components, myosin/myosin light-chain kinase, spindle stability regulators, and spindle assembly checkpoint regulators, play critical roles in asymmetric cell division in oocytes. This review summarizes recent findings on these actin-related regulators in mammalian oocyte asymmetric division and outlines a complete signaling pathway.

Calcium signaling in mammalian egg activation and embryo development: the influence of subcellular localization

Calcium (Ca(2+) ) signals drive the fundamental events surrounding fertilization and the activation of development in all species examined to date. Initial studies of Ca(2+) signaling at fertilization in marine animals were tightly linked to new discoveries of bioluminescent proteins and their use as fluorescent Ca(2+) sensors. Since that time, there has been rapid progress in our understanding of the key functions for Ca(2+) in many cell types and of the impact of cellular localization on Ca(2+) signaling pathways. In this review, which focuses on mammalian egg activation, we consider how Ca(2+) is regulated and stored at different stages of oocyte development and examine the functions of molecules that serve as both regulators of Ca(2+) release and effectors of Ca(2+) signals. We then summarize studies exploring how Ca(2+) directs downstream effectors mediating both egg activation and later signaling events required for successful preimplantation embryo development. Throughout this review, we focus attention on how localization of Ca(2+) signals influences downstream signaling events, and attempt to highlight gaps in our knowledge that are ripe for future research.

MARCKS actin-binding capacity mediates actin filament assembly during mitosis in human hepatic stellate cells

Cross-linking between the actin cytoskeleton and plasma membrane actin-binding proteins is a key interaction responsible for the mechanical properties of the mitotic cell. Little is known about the identity, the localization, and the function of actin filament-binding proteins during mitosis in human hepatic stellate cells (hHSC). The aim of the present study was to identify and analyze the cross talk between actin and myristoylated alanine-rich kinase C substrate (MARCKS), an important PKC substrate and actin filament-binding protein, during mitosis in primary hHSC. Confocal analysis and chromosomal fraction analysis of mitotic hHSC demonstrated that phosphorylated (P)-MARCKS displays distinct phase-dependent localizations, accumulates at the perichromosomal layer, and is a centrosomal protein belonging to the chromosomal cytosolic fraction. Aurora B kinase (AUBK), an important mitotic regulator, β-actin, and P-MARCKS concentrate at the cytokinetic midbody during cleavage furrow formation. This localization is critical since MARCKS-depletion in hHSC is characterized by a significant loss in cytosolic actin filaments and cortical β-actin that induces cell cycle inhibition and dislocation of AUBK. A depletion of AUBK in hHSC affects cell cycle, resulting in multinucleation. Quantitative live cell imaging demonstrates that the actin filament-binding capacity of MARCKS is key to regulate mitosis since the cell cycle inhibitory effect in MARCKS-depleted cells caused abnormal cell morphology and an aberrant cytokinesis, resulting in a significant increase in cell cycle time. These findings implicate that MARCKS, an important PKC substrate, is essential for proper cytokinesis and that MARCKS and its partner actin are key mitotic regulators during cell cycle in hHSC.

PhosphoMARCKS drives motility of mouse melanoma cells

Phosphorylation of myristoylated alanine-rich C-kinase substrate (MARCKS) by protein kinase C alpha (PKC alpha) is known to trigger its release from the plasma membrane/cytoskeleton into the cytoplasm, thereby promoting actin reorganization during migration. This study shows that once released into the cytoplasm, phosphoMARCKS directly promotes motility of melanoma cells. Aggressively motile B16 F10 mouse melanoma cells express high levels of phosphoMARCKS, whereas in weakly motile B16 F1 cells it is undetectable. Following treatment with okadaic acid (OA) (a protein phosphatase inhibitor), F1 cells exhibited a dramatic increase in phosphoMARCKS that was co-incident with a 5-fold increase in motility. Both MARCKS phosphorylation and motility were substantially decreased when prior to OA addition, MARCKS expression was knocked out by a MARCKS-specific shRNA, thereby implicating MARCKS as a major component of the motility pathway. Decreased motility and phosphoMARCKS levels in OA-treated cells were observed with a PKC inhibitor (calphostin C), thus indicating that PKC actively phosphorylates MARCKS in F1 cells but that this reaction is efficiently reversed by protein phosphatases. The mechanistic significance of phosphoMARCKS to motility was further established with a pseudo-phosphorylated mutant of MARCKS-GFP in which Asp residues replaced Ser residues known to be phosphorylated by PKC alpha. This mutant localized to the cytoplasm and engendered three-fold higher motility in F1 cells. Expression of an unmyristoylated, phosphorylation-resistant MARCKS mutant that localized to the cytoplasm, blocked motility by 40-50% of both OA-stimulated F1 cells and intrinsically motile F10 cells. These results demonstrate that phosphoMARCKS contributes to the metastatic potential of melanoma cells, and reveal a previously undocumented signaling role for this cytoplasmic phospho-protein.

Aurora kinase B modulates chromosome alignment in mouse oocytes

The elevated incidence of aneuploidy in human oocytes warrants study of the molecular mechanisms regulating proper chromosome segregation. The Aurora kinases are a well-conserved family of serine/threonine kinases that are involved in proper chromosome segregation during mitosis and meiosis. Here we report the expression and localization of all three Aurora kinase homologs, AURKA, AURKB, and AURKC, during meiotic maturation of mouse oocytes. AURKA, the most abundantly expressed homolog, localizes to the spindle poles during meiosis I (MI) and meiosis II (MII), whereas AURKB is concentrated at kinetochores, specifically at metaphase of MI (Met I). The germ cell-specific homolog, AURKC, is found along the entire length of chromosomes during both meiotic divisions. Maturing oocytes in the presence of the small molecule pan-Aurora kinase inhibitor, ZM447439 results in defects in meiotic progression and chromosome alignment at both Met I and Met II. Over-expression of AURKB, but not AURKA or AURKC, rescues the chromosome alignment defect suggesting that AURKB is the primary Aurora kinase responsible for regulating chromosome dynamics during meiosis in mouse oocytes.

Deubiquitinating enzyme USP33/VDU1 is required for Slit signaling in inhibiting breast cancer cell migration

Slit regulates migration of not only neurons, but also nonneuronal cells, such as leukocytes and cancer cells. Slit effect on cancer cell migration has not been well-characterized. In this study, we used several different assays to examine Slit effect on breast cancer cell migration in vitro. We show that ubiquitin-specific protease 33 (USP33)/VDU1, originally identified as a von Hippel-Lindau tumor suppressor (VHL) protein-interacting deubiquitinating enzyme, binds to the Robo1 receptor, and that USP33 is required for Slit responsiveness in breast cancer cells. Slit induces redistribution of Robo1 from intracellular compartments to the plasma membrane in a USP33-dependent manner. Slit impairs directional migration of breast cancer cells without affecting their migration speed. This inhibitory effect is Robo-mediated and USP33-dependent. These data uncover a previously unknown function of USP33 and reveal a new player in Slit-Robo signaling in cancer cell migration.

Actin filaments: key players in the control of asymmetric divisions in mouse oocytes

Meiotic maturation is characterized by the succession of two asymmetric divisions each giving rise to a small polar body and a large oocyte. These highly asymmetric divisions are characteristic of meiosis in higher organisms. They allow most of the maternal stores to be retained in the oocyte, a vital property for further embryo development. In mouse oocytes, the asymmetry is ensured by the migration and the anchoring of the division spindle to the cortex in meiosis I and by its anchoring to the cortex in meiosis II. In addition, and subsequent to this off-centre positioning of the spindle, a differentiation of the cortex overhanging the chromosomes takes place and is necessary for the extrusion of small polar bodies. In the present review, we will emphasize the role of the actin cytoskeleton in the control of spindle positioning, spindle anchoring to the cortex and cortical differentiation.

Protein kinase C delta (PKCdelta) interacts with microtubule organizing center (MTOC)-associated proteins and participates in meiotic spindle organization

Defects in meiotic spindle structure can lead to chromosome segregation errors and genomic instability. In this study the potential role of protein kinase C delta (PKCdelta) on meiotic spindle organization was evaluated in mouse oocytes. PKCdelta was previously shown to be phosphorylated during meiotic maturation and concentrate on the meiotic spindle during metaphases I and II. Currently we show that when phosphorylated on Threonine 505 (pPKCdelta(Thr505)), within the activation loop of its C4 domain, PKCdelta expression was restricted to the meiotic spindle poles and a few specific cytoplasmic foci. In addition, pPKCdelta(Thr505) co-localized with two key microtubule organizing center (MTOC)-associated proteins, pericentrin and gamma-tubulin. An interaction between pPKCdelta(Thr505) and pericentrin as well as gamma-tubulin was confirmed by co-immunoprecipitation analysis using both fetal fibroblast cells and oocytes. Notably, targeted knockdown of PKCdelta expression in oocytes using short interfering RNAs effectively reduced pPKCdelta(Thr505) protein expression at MTOCs and leads to a significant (P < 0.05) disruption of meiotic spindle organization and chromosome alignment during MI and MII. Moreover, both gamma-tubulin and pericentrin expression at MTOCs were decreased in pPKCdelta(Thr505)-depleted oocytes. In sum, these results indicate that pPKCdelta(Thr505) interacts with MTOC-associated proteins and plays a role in meiotic spindle organization in mammalian oocytes.

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.

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.

Embryologic, cytobiologic and genetic interpretations of DDK syndrome in mice

DDK syndrome is known as embryonic death at the morula-blastocyst stage in female mice of the DDK strain mated with males from other strains (alien males). The embryonic death is interpreted to be caused by incompatibility between oocyte factors and the product from male pronucleus, both of which are under the control of alleles at the same locus on Chromosome 11. This review explains the hypothesis proposing that the embryonic death may be caused primarily by failure in de novo regeneration of centrosomes containing centrioles in the trophectodermal cells. Centrioles disintegrate during gametogenesis in mice, and new centrioles are formed after the cleavage stage during which cell division proceeds with the microtubule organizing center having no centrioles. The failure in de novo regeneration of the centrosomes may arrest cell division and consequently result in embryonic death. Another aspect of DDK syndrome is distortion of the second polar body extrusion in the semi-incompatible cross. In the heterozygous (DDK/alien) oocytes fertilized with alien spermatozoa, DDK allele is more frequently retained in the oocyte nucleus, and alien allele tends to be carried into the polar body. This distortion may possibly be caused by derangement in the spindle system. Therefore, both aspects of DDK syndrome can be regarded as being derived from the abnormality in the centrosome-spindle system according to this hypothesis.

Proline-rich tyrosine kinase2 is involved in F-actin organization during in vitro maturation of rat oocyte

Microfilaments (actin filaments) regulate various dynamic events during meiotic maturation. Relatively, little is known about the regulation of microfilament organization in mammalian oocytes. Proline-rich tyrosine kinase2 (Pyk2), a protein tyrosine kinase related to focal adhesion kinase (FAK) is essential in actin filaments organization. The present study was to examine the expression and localization of Pyk2, and in particular, its function during rat oocyte maturation. For the first time, by using Western blot and confocal laser scanning microscopy, we detected the expression of Pyk2 in rat oocytes and found that Pyk2 and Try402 phospho-Pyk2 were localized uniformly at the cell cortex and surrounded the germinal vesicle (GV) or the condensed chromosomes at the GV stage or after GV breakdown. At the metaphase and the beginning of anaphase, Pyk2 distributed asymmetrically both in the ooplasm and the cortex with a marked staining associated with the chromosomes and the region overlying the meiotic spindle. At telophase, Pyk2 was observed in the cleavage furrows in addition to its cortex and cytoplasm localization. The dynamics of Pyk2 were similar to that of F-actin, and this kinase was found to co-localize with microfilaments in several developmental stages during rat oocyte maturation. Microinjection of Pyk2 antibody demolished the microfilaments assembly and also inhibited the first polar body (PB1) emission. These findings suggest an important role of Pyk2 for rat oocyte maturation by regulating the organization of actin filaments.

2-methoxyestradiol induces spindle aberrations, chromosome congression failure, and nondisjunction in mouse oocytes

2-Methoxyestradiol (2-ME) is a metabolite of 17beta-estradiol and a natural component of follicular fluid. Local concentrations of 2-ME may be increased by exposure to environmental pollutants that activate the expression of enzymes in the metabolic pathway from 17beta-estradiol to 2-ME. It has been suspected that this may have adverse effects on spindle formation in maturing oocytes, which would affect embryo quality. To study the dose-response patterns, we exposed denuded mouse oocytes to 2-ME during in vitro maturation. Meiotic progression, spindle morphology, centrosome integrity, and chromosome congression were examined by immunofluorescence and noninvasive polarizing microscopy (PolScope). Chromosomal constituents were assessed after spreading and C-banding. 2-ME sustained MAD2L1 expression at the centromeres and increased the number of meiosis I-blocked oocytes in a dose-dependent manner. 2-ME also caused dramatic dose-dependent increases in the hyperploidy of metaphase II oocytes. Some of these meiosis II oocytes contained anaphase I-like chromosomes, which suggests that high concentrations of the catecholestradiol interfere with the physical separation of chromosomes. Noninvasive PolScope analysis and tubulin immunofluorescence revealed that perturbations in spindle organization, which resulted in severe disturbances of the chromosome alignment at the spindle equator (congression failure), were caused by 2-ME at meiosis I and II. Pericentrin-positive centrosomes failed to align at the spindle poles, and multipolar spindles and prominent arrays of cytoplasmic microtubule asters were induced in 2-ME-exposed metaphase II oocytes. In conclusion, a micromolar level of 2-ME is aneugenic for mammalian oocytes. Therefore, exposure to 2-ME and conditions that increase the intrinsic local concentration of 2-ME in the ovary may affect fertility and increase risks for chromosomal aberrations in the oocyte and embryo.

Asymmetric divisions of germline cells

In most vertebrates and invertebrates, germ cells produce female and male gametes after one or several rounds of asymmetric cell division. Germline-specific features are used for the asymmetric segregation of fates, chromosomes and size during gametogenesis. In Drosophila females, for example, a germline-specific organelle called the fusome is used repeatedly to polarize the divisions of germline stem cells for their self-renewal, and during the divisions of cyst cells for the specification of the oocyte among a group of sister cells sharing a common cytoplasm. Later during oogenesis of most species, meiotic divisions produce a striking size asymmetry between a large oocyte and small polar bodies. The strategy used to create this asymmetry may involve the microtubules or the actin microfilaments or both, depending on the considered species. Despite this diversity and species-particularities, recent molecular data suggest that the PAR proteins, which control asymmetric cell division in a wide range of organisms and somatic cell types, could also play an important role at different steps of gametogenesis in many species. Here, we review the asymmetric features of germline cell division, from mitosis of germline stem cells to the extrusion of polar bodies after meiotic divisions.

Regulation of dynamic events by microfilaments during oocyte maturation and fertilization

Actin filaments (microfilaments) regulate various dynamic events during oocyte meiotic maturation and fertilization. In most species, microfilaments are not required for germinal vesicle breakdown and meiotic spindle formation, but they mediate peripheral nucleus (chromosome) migration, cortical spindle anchorage, homologous chromosome separation, cortex development/maintenance, polarity establishment, and first polar body emission during oocyte maturation. Peripheral cortical granule migration is controlled by microfilaments, while mitochondria movement is mediated by microtubules. During fertilization, microfilaments are involved in sperm incorporation, spindle rotation (mouse), cortical granule exocytosis, second polar body emission and cleavage ring formation, but are not required for pronuclear apposition (except for the mouse). Many of the events are driven by the dynamic interactions between myosin and actin filaments whose polymerization is regulated by RhoA, Cdc42, Arp2/3 and other signaling molecules. Studies have also shown that oocyte cortex organization and polarity formation mediated by actin filaments are regulated by mitogen-activated protein kinase, myosin light-chain kinase, protein kinase C and its substrate p-MARKS as well as PAR proteins. The completion of several dynamic events, including homologous chromosome separation, spindle anchorage, spindle rotation, vesicle organelle transport and pronuclear apposition (mouse), requires interactions between microfilaments and microtubules, but determination of how the two systems of the cytoskeleton precisely cross-link, and which proteins link microfilaments to microtubules to perform functions in eggs, requires further studies. Finally, the meaning of microfilament-mediated oocyte polarity versus embryo polarity and embryo development in different species (Drosophila, Xenopus and mouse) is discussed.

Myoblast attachment and spreading are regulated by different patterns by ubiquitous calpains

The calcium-dependent proteolytic system is a large family of well-conserved ubiquitous and tissue-specific proteases, known as calpains, and an endogenous inhibitor, calpastatin. Ubiquitous calpains are involved in many physiological phenomena, such as the cell cycle, muscle cell differentiation, and cell migration. This study investigates the regulation of crucial steps of cell motility, myoblast adhesion and spreading, by calpains. Inhibition of each ubiquitous calpain isoform by antisense strategy pinpointed the involvement of each of these proteases in myoblast adhesion and spreading. Moreover, the actin cytoskeleton and microtubules were observed in transfected cells, demonstrating that each ubiquitous calpain could be involved in the actin fiber organization. C2C12 cells with reduced mu- or m-calpain levels have a rounded morphology and disorganized stress fibers, but no modification in the microtubule cytoskeleton. Antisense strategy directed against MARCKS, a calpain substrate during C2C12 migration, showed that this protein could play a role in stress fiber polymerization. A complementary proteomic analysis using C2C12 cells over-expressing calpastatin indicated that two proteins were under-expressed, while six, which are involved in the studied phenomena, were overexpressed after calpain inhibition. The possible role of these proteins in adhesion, spreading, and migration was discussed.

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.

Establishment of animal–vegetal polarity during maturation in ascidian oocytes

Mature ascidian oocytes are arrested in metaphase of meiosis I (Met I) and display a pronounced animal-vegetal polarity: a small meiotic spindle lies beneath the animal pole, and two adjacent cortical and subcortical domains respectively rich in cortical endoplasmic reticulum and postplasmic/PEM RNAs (cER/mRNA domain) and mitochondria (myoplasm domain) line the equatorial and vegetal regions. Symmetry-breaking events triggered by the fertilizing sperm remodel this primary animal-vegetal (a-v) axis to establish the embryonic (D-V, A-P) axes. To understand how this radial a-v polarity of eggs is established, we have analyzed the distribution of mitochondria, mRNAs, microtubules and chromosomes in pre-vitellogenic, vitellogenic and post-vitellogenic Germinal Vesicle (GV) stage oocytes and in spontaneously maturing oocytes of the ascidian Ciona intestinalis. We show that myoplasm and postplasmic/PEM RNAs move into the oocyte periphery at the end of oogenesis and that polarization along the a-v axis occurs after maturation in several steps which take 3-4 h to be completed. First, the Germinal Vesicle breaks down, and a meiotic spindle forms in the center of the oocyte. Second, the meiotic spindle moves in an apparently random direction towards the cortex. Third, when the microtubular spindle and chromosomes arrive and rotate in the cortex (defining the animal pole), the subcortical myoplasm domain and cortical postplasmic/PEM RNAs are excluded from the animal pole region, thus concentrating in the vegetal hemisphere. The actin cytoskeleton is required for migration of the spindle and subsequent polarization, whereas these events occur normally in the absence of microtubules. Our observations set the stage for understanding the mechanisms governing primary axis establishment and meiotic maturation in ascidians.

Essential Role for the PKC Target MARCKS in Maintaining Dendritic Spine Morphology

Spine morphology is regulated by intracellular signals, like PKC, that affect cytoskeletal and membrane dynamics. We investigated the role of MARCKS (myristoylated, alanine-rich C-kinase substrate) in dendrites of 3-week-old hippocampal cultures. MARCKS associates with membranes via the combined action of myristoylation and a polybasic effector domain, which binds phospholipids and/or F-actin, unless phosphorylated by PKC. Knockdown of endogenous MARCKS using RNAi reduced spine density and size. PKC activation induced similar effects, which were prevented by expression of a nonphosphorylatable mutant. Moreover, expression of pseudophosphorylated MARCKS was, by itself, sufficient to induce spine loss and shrinkage, accompanied by reduced F-actin content. Nonphosphorylatable MARCKS caused spine elongation and increased the mobility of spine actin clusters. Surprisingly, it also decreased spine density via a novel mechanism of spine fusion, an effect that required the myristoylation sequence. Thus, MARCKS is a key factor in the maintenance of dendritic spines and contributes to PKC-dependent morphological plasticity.