Localized calcium signals in early zebrafish development.

Activation of the phosphoinositide (PI) pathway has been shown to be involved in the compaction of blastomeres in mouse embryos and in embryonic axis formation in Xenopus and in zebrafish embryos. Here we investigate Ca2+ signals in individual blastomeres of zebrafish embryos with the goal to better understand the role of PI and Ca2+ signaling for early vertebrate embryogenesis. Initial studies showed that the inositol 1,4,5-trisphosphate (IP3) concentration increases after the 32-cell stage of development, suggesting that IP3-mediated Ca2+ signals may be present during the blastula stage. Ca2+ signals were measured by identifying individual cells using confocal imaging of a nuclear localized Ca2+ indicator. Using this in situ indicator, changes in Ca2+ concentration were measured over several hours in each cell of a series of sections through the developing embryo. Transient increases in Ca2+ concentration that lasted 20-50 sec (Ca2+ spikes) were first triggered during the 32- to 128-cell stage in cells of the outer embryonic cell layer. These cells develop epithelial characteristics and specialize into the enveloping layer (EVL). No Ca2+ activity was observed during the earlier cleavage cycles or in deep blastomeres. Ca2+ spikes remained restricted to the EVL until the end of the blastula stage. Ca2+ spikes in neighboring EVL cells often occurred in the same short time interval, indicating that small groups of EVL cells can synchronize their activity. When averaged over several cell cycles, Ca2+ activity showed an even distribution in the EVL and did not indicate future polarities.

Accurate in vitro cleavage by RNase III of phosphorothioate-substituted RNA processing signals in bacteriophage T7 early mRNA.

To test the ability of an RNA processing enzyme to cleave chemically-modified RNA substrates, RNA transcripts containing RNase III cleavage sites were enzymatically synthesized in vitro to contain specific phosphorothioate diester internucleotide linkages. One transcript (R1.1 RNA) was generated using phage T7 RNA polymerase and a cloned segment of phage T7 DNA containing the R1.1 RNase III processing site. The second transcript was the phage T7 polycistronic early mRNA precursor, which was synthesized using E. coli RNA polymerase and T7 genomic DNA. The RNA transcripts contained phosphorothioate diester groups at positions including the scissile bonds. The modified RNAs were stable to incubation in Mg2+-containing buffer, and were specifically cleaved by RNase III. RNA oligonucleotide sequence analysis showed that the modified R1.1 RNA processing site was the same as the canonical site and contained a phosphorothioate bond. Furthermore, RNase III cleaved the phosphorothioate internucleotide bond with 5' polarity. RNase III cleavage of phosphorothioate substituted T7 polycistronic early mRNA precursor produced the same gel electrophoretic pattern as that obtained with the control transcript. Thus, RNase III cleavage specificity is not altered by phosphorothioate internucleotide linkages.

Myosin subfragment-1 is sufficient to move actin filaments in vitro.

The rotating crossbridge model for muscle contraction proposes that force is produced by a change in angle of the crossbridge between the overlapping thick and thin filaments. Myosin, the major component of the thick filament, is comprised of two heavy chains and two pairs of light chains. Together they form two globular heads, which give rise to the crossbridge in muscle, and a coiled-coil rod, which forms the shaft of the thick filament. The isolated head fragment, subfragment-1 (S1), contains the ATPase and actin-binding activities of myosin (Fig. 1). Although S1 seems to have the requisite enzymatic activity, direct evidence that S1 is sufficient to drive actin movement has been lacking. It has long been recognized that in vitro movement assays are an important approach for identifying the elements in muscle responsible for force generation. Hynes et al. showed that beads coated with heavy meromyosin (HMM), a soluble proteolytic fragment of myosin consisting of a part of the rod and the two heads, can move on Nitella actin filaments. Using the myosin-coated surface assay of Kron and Spudich, Harada et al. showed that single-headed myosin filaments bound to glass support movement of actin at nearly the same speed as intact myosin filaments. These studies show that the terminal portion of the rod and the two-headed nature of myosin are not required for movement. To restrict the region responsible for movement further, we have modified the myosin-coated surface assay by replacing the glass surface with a nitrocellulose film. Here we report that myosin filaments, soluble myosin, HMM or S1, when bound to a nitrocellulose film, support actin sliding movement (Fig. 2). That S1 is sufficient to cause sliding movement of actin filaments in vitro gives strong support to models of contraction that place the site of active movement in muscle within the myosin head.