Injection of an antibody against a p21 c-Ha-ras protein inhibits cleavage in axolotl eggs

The wild-type Ras: road ahead

The cellular Ras is known to play an important role in cellular proliferation mediated by growth factor receptor. Evidence also points to its role in growth arrest. Substantiated proof for growth-suppressive activity of wild-type Ras comes from studies that showed 1) loss of wild-type ras allele in tumors, 2) suppression of growth in cells transformed by oncogenic ras upon overexpression of wild-type Ras, and 3) up-regulation of Ras expression during postnatal development and following growth arrest of untransformed cells in culture. To understand the mechanism by which the wild-type Ras brings about these diverse actions, we evaluated its well-known role in actively proliferating cells and its less understood role in growth arrest. This led to the proposal that wild-type Ras in either GDP or GTP-bound state can antagonize the function of oncogenic Ras.-Singh, A., Sowjanya, A. P., Ramakrishna, G. The wild-type Ras: road ahead.

Reducing inositol lipid hydrolysis, Ins(1,4,5)P3 receptor availability, or Ca2+ gradients lengthens the duration of the cell cycle in Xenopus laevis blastomeres

We have microinjected a mAb specifically directed to phosphatidylinositol 4,5-bisphosphate (PIP2) into one blastomere of two-cell stage Xenopus laevis embryos. This antibody binds to endogenous PIP2 and reduces its rate of hydrolysis by phospholipase C. Antibody-injected blastomeres undergo partial or complete arrest of the cell cycle whereas the uninjected sister blastomeres divided normally. Since PIP2 hydrolysis normally produces diacylglycerol (DG) and inositol 1,4,5-triphosphate (Ins[1,4,5]P3), we attempted to measure changes in the levels of DG following stimulation of PIP2 hydrolysis in antibody-injected oocytes. The total amount of DG in antibody-injected oocytes was significantly reduced compared to that of water-injected ones following stimulation by either acetylcholine or progesterone indicating that the antibody does indeed suppress PIP2 hydrolysis. We also found that the PIP2 antibodies greatly reduced the amount of intracellular Ca2+ released in the egg cortex during egg activation. As an indirect test for Ins(1,4,5)P3 involvement in the cell cycle we injected heparin which competes with Ins(1,4,5)P3 for binding to its receptor, and thus inhibits Ins(1,4,5)P3-induced Ca2+ release. Microinjection of heparin into one blastomere of the two-cell stage embryo caused partial or complete arrest of the cell cycle depending upon the concentration of heparin injected. We further investigated the effect of reducing any [Ca2+]i gradients by microinjecting dibromo-BAPTA into the blastomere. Dibromo-BAPTA injection completely blocked mitotic cell division when a final concentration of 1.5 mM was used. These results suggest that PIP2 turnover as well as second messenger activity influence cell cycle duration during embryonic cell division in frogs.

Dominant inhibitory Ras mutants selectively inhibit the activity of either cellular or oncogenic Ras

Two dominant inhibitory Ras mutant proteins were analyzed by microinjection. One, [Asn-17]Ras, had a substitution in the putative Mg(2+)-binding site of Ha-Ras. The other, RAST, had a mutation in a yeast RAS protein that impaired its GTPase activity and increased its affinity for GAP. RAST also had a mutation that blocked its localization to the plasma membrane. In NIH 3T3 cells [Asn-17]Ras inhibited the function of normal Ras much more efficiently than that of oncogenic Ras. In contrast, RAST interfered with the transforming activity of oncogenic Ras more efficiently than that of normal Ras. These conclusions were based on two separate types of analysis. The inhibitory Ras mutant proteins were first microinjected into cells stably transformed either by oncogenic Ras or by high levels of expression of cellular Ras. Results obtained in stably transformed cells were then verified by coinjection of the inhibitory Ras mutant proteins together with transforming concentrations of either oncogenic or normal Ras protein. Whereas RAST was active in soluble form. [Asn-17]Ras required membrane localization for activity. Furthermore, mutations in the GAP/effector-binding domain reduced or eliminated the inhibitory activity of RAST but had no detectable effect on [Asn-17]Ras. These results are consistent with the possibility that [Asn-17]Ras functions by blocking the activation of endogenous Ras proteins, while RAST functions by blocking the ability of activated Ras to stimulate a downstream target within the cells. The properties of RAST suggest that interference with the GAP/effector-binding function of RAS represents a strategy for the preferential inactivation of oncogenic Ras in cells.

Dominant inhibitory Ras mutants selectively inhibit the activity of either cellular or oncogenic Ras

Two dominant inhibitory Ras mutant proteins were analyzed by microinjection. One, [Asn-17]Ras, had a substitution in the putative Mg(2+)-binding site of Ha-Ras. The other, RAST, had a mutation in a yeast RAS protein that impaired its GTPase activity and increased its affinity for GAP. RAST also had a mutation that blocked its localization to the plasma membrane. In NIH 3T3 cells [Asn-17]Ras inhibited the function of normal Ras much more efficiently than that of oncogenic Ras. In contrast, RAST interfered with the transforming activity of oncogenic Ras more efficiently than that of normal Ras. These conclusions were based on two separate types of analysis. The inhibitory Ras mutant proteins were first microinjected into cells stably transformed either by oncogenic Ras or by high levels of expression of cellular Ras. Results obtained in stably transformed cells were then verified by coinjection of the inhibitory Ras mutant proteins together with transforming concentrations of either oncogenic or normal Ras protein. Whereas RAST was active in soluble form. [Asn-17]Ras required membrane localization for activity. Furthermore, mutations in the GAP/effector-binding domain reduced or eliminated the inhibitory activity of RAST but had no detectable effect on [Asn-17]Ras. These results are consistent with the possibility that [Asn-17]Ras functions by blocking the activation of endogenous Ras proteins, while RAST functions by blocking the ability of activated Ras to stimulate a downstream target within the cells. The properties of RAST suggest that interference with the GAP/effector-binding function of RAS represents a strategy for the preferential inactivation of oncogenic Ras in cells.

Expression of ras-like proteins in embryonic and adult cells of Xenopus laevis

A polyclonal antibody raised against v-Ha-ras p21 was purified and its specificity was checked on Ha-ras transformed cell lines. It was used to immunoprecipitate p21 from different Xenopus laevis cell types: brain cells, blood cells, and embryonic material. By one-dimensional Western blot analysis, we show that ras p21 is synthesized very early in oogenesis and accumulates throughout vitellogenesis. The ras p21 content, estimated to be 1.1 ng in the full-grown oocyte, remains constant during oocyte maturation and egg cleavage. Increase in the amount of ras p21 occurs at the beginning of neurulation. Two-dimensional Western blot patterns reveal the presence of multiple molecular forms of p21 in all Xenopus cell types studied. The numerous resolved polypeptides were ascribed to the expression of at least two different ras genes. Furthermore, specific charge modifications of the ras polypeptides are observed in brain, blood, and embryonic cells. During oogenesis and early embryonic development, differences in two-dimensional patterns mainly concern variations in the relative amounts of the different polypeptides. The results are discussed in relation to the well documented synthesis activities of the growing oocyte and of the early developing embryo.

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.

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.

Cytological effects of the microinjection of antibody to ras p21 in early cleavage Xenopus embryos

The presence of two ras-related proteins (22 and 23 kDa) was demonstrated in Xenopus embryonic extracts by selective immunoprecipitation using anti-ras monoclonal antibodies 142-24E05 and Y13-259. We further describe the cytological effects of the microinjection of anti-ras monoclonal antibody Y13-259 into early cleavage blastomeres of Xenopus embryos. Injection of the antibody into a blastomere at the two-, four-, or eight-cell stage caused cleavage arrest in the descendants of the injected blastomere. Light microscopy (LM) of cleavage-arrested cells revealed extensive deformation of the cells as well as heterogeneity of distribution of yolk platelets and pigment granules. LM analysis of serial sections of cleavage-arrested cells revealed the presence of multiple nuclei. Although the nuclei expressed similar morphological properties, indicating that they were probably in the same stage of the nuclear cycle, they revealed highly variable chromatin densities. Electron microscope (EM) analysis of the cytoplasm of cleavage-arrested cells revealed the accumulation of vesicles and large membranous elements coincident with cleavage arrest. Furthermore, endoplasmic reticulum (ER) existed in two forms, as closed, circular profiles and as long, linear arrays. Mitochondria were characteristically aligned in single file on both sides of the two types of ER cisternae. EM analysis of nuclei confirmed variations in chromatin organization and suggested the occurrence of unique nuclear envelope fusion among micronuclei in cleavage-arrested cells. Cleavage arrest and changes in cytological features were not observed in the cytoplasm of cells microinjected with normal rat IgG. Thus the immunochemical data and microinjection experiments suggest that ras-like or ras antigenicity exists within rapidly replicating Xenopus blastomeres and may be involved in the organization of a number of its cytoplasmic elements.

Function of c-mos proto-oncogene product in meiotic maturation in Xenopus oocytes

The c-mos proto-oncogene is expressed as a maternal mRNA in oocytes and early embryos of Xenopus laevis, but its translation product pp39mos is detectable only during progesterone-induced oocyte maturation. Microinjection of mos-specific antisense oligonucleotides into oocytes not only prevents expression of pp39mos, but also blocks germinal vesicle breakdown, indicating that it functions during reinitiation of meiotic division.