Drosophila phospholipase C-gamma expressed predominantly in blastoderm cells at cellularization and in endodermal cells during later embryonic stages.

A Drosophila gene encoding a gamma-type isozyme of phosphoinositide-specific phospholipase C (PLC) was isolated and characterized. The gene, termed plc-gamma d, was mapped at position 14B-C of the X chromosome. The encoded protein, termed PLC-gamma D, contains X and Y regions, common to all known PLC isozymes. The two regions are split by a Z region that comprises two src homology 2 and one src homology 3 domains and is characteristic of gamma-type mammalian PLC (PLC-gamma 1 and -gamma 2). The deduced amino acid sequence of PLC-gamma D shows overall similarity to mammalian PLC-gamma s; no large deletion was observed except the short C-terminal extended region. In particular, the two split catalytic domains (X and Y regions) and the regulatory Z region including the src homology 2 and src homology 3 domains are well conserved. The mRNA is expressed throughout development, but expression is relatively higher during the embryonic stage, suggesting fundamental and important roles in both cell proliferation and differentiation. Distribution of the mRNA during embryogenesis, as analyzed by whole amount in situ hybridization, revealed that the mRNA emerges and reaches maximum levels at the cellular blastoderm stage and then decreases rapidly to a lower level. In later embryonic stages, invaginated anterior and posterior midgut primordia show high levels of mRNA expression, and fused midgut also maintains a high level of expression. In other tissues and cells, the mRNA was detected at lower levels. These results indicate that Drosophila PLC-gamma may be involved in universal cellular processes mediated possibly by receptor tyrosine kinases during embryogenesis and may also play specific roles during cellularization and midgut differentiation.

A Drosophila homolog of human proto-oncogene ret transiently expressed in embryonic neuronal precursor cells including neuroblasts and CNS cells.

We have identified a Drosophila gene encoding a putative receptor tyrosine kinase by screening a genomic DNA library with a DNA probe for a Drosophila homolog of fibroblast growth factor receptors. The newly isolated gene codes for a transmembrane protein most similar in sequence to a mammalian proto-oncogene ret; thus, the gene was termed Dret. Dret mRNA is transcribed in very small amounts in the embryonic, larval, and pupal stages. Whole mount in situ hybridization experiments revealed that the mRNA is transiently expressed in neuroblasts in early embryos. In late embryos, Dret mRNA was detected in subpopulations of differentiating CNS and PNS cells. In addition, Dret expression was affected in neurogenic mutants. These results suggest that Dret can be considered as a functional homolog of mammalian ret and should play important roles in neurogenesis.

Identification of a novel Drosophila gene encoding a Cdc2-related protein kinase.

We have identified a novel gene encoding a putative protein kinase from a Drosophila genomic library. The gene, about 2 kbp in length, consists of four exons and codes for a protein of 349 amino acid residues. The deduced sequence shows significant similarity to various kinases, especially to a subgroup of Ser/Thr kinases related to Cdc2 kinase; thus, the gene was termed Dcdrk (Drosophila cdc2-related kinase gene). Among the kinases examined, mammalian galactosyltransferase-associated 58 kDa protein kinase showed the highest homology (about 50% identity in the kinase domain) to Dcdrk kinase. Northern blot analysis revealed that the Dcdrk mRNA is expressed throughout development in nearly constant amounts. Moreover, a whole mount in situ hybridization experiment showed that the Dcdrk mRNA is ubiquitously distributed in almost all embryonic cells and tissues, suggesting a universal function of Dcdrk, possibly in cell cycle regulation.

Force-velocity relation of frog skeletal muscle fibres shortening under continuously changing load.

1. The force-velocity (P-V) relation of single frog skeletal muscle fibres was examined in non-steady-state conditions in which the load on the fibre changed continuously with time. Two such types of condition were used; in one type, the load was increased from zero to the maximum isometric force (P0) (auxotonic condition), while in the other type the load was decreased form P0 to zero at constant rates (ramp decrease in load). 2. The P-V curves obtained in the auxotonic condition were convex upwards and always below the hyperbolic P-V curve obtained in the isotonic condition. Different curves were obtained depending on the compliance of auxotonic load. The shortening velocity for a given amount of load increased with increasing compliance. 3. Qualitatively similar P-V relations were obtained irrespective of whether the fibre was made to shorten auxotonically at the onset of stimulation or after the development of P0. 4. If the force at any time after the onset of auxotonic shortening was normalized relative to the isometric force at the same time after the onset of isometric force development, the normalized force versus velocity curves were found to fit well to the hyperbolic P-V curve in the isotonic condition except for the low-force region. 5. The P-V curves obtained during the ramp decrease in load were hyperbolic in shape except for the humps at the high-force region and always above the P-V curve in the isotonic condition. Different curves were obtained depending on the rate of load decrease. The maximum shortening velocity increased with increasing rate of load decrease. 6. The above features of the P-V relations could well be simulated by a simplified Huxley contraction model, indicating that the kinetic properties of the cross-bridges are the same in both steady- and non-steady-state conditions.

Economic force maintenance in a phasic smooth muscle of mollusca.

The time course of the ability of active shortening was compared to that of isometric contractile force during contraction-relaxation cycles of the pedal retractor muscle of Mytilus by measuring the maximal shortening velocity and the rate of force redevelopment after a quick release. The contractile force had its peak at 2 s after stimulus initiation and decayed with a half relaxation time of 8.3 s at 10 degrees C. At 6 s after stimulus initiation, both the maximal velocity of shortening and the rate of force redevelopment had decreased to 30% or less of the initial values, while 70% of peak force was retained. The results suggest a mechanism of economic force maintenance with a reduced rate of crossbridge cycling.