Gastrulation is the process by which the germ layers become positioned in an embryo. C. elegans gastrulation serves as a model for studying the molecular mechanisms of diverse cellular and developmental phenomena, including morphogenesis, cell polarization, cell-cell signaling, actomyosin contraction and cell-cell adhesion. One distinct advantage of studying these phenomena in C. elegans is that genetic tools can be combined with high resolution live cell imaging and direct manipulations of the cells involved. Here we review what is known to date about the cellular and molecular mechanisms that function in C. elegans gastrulation.

We conclude that Wnt signaling regulates C. elegans gastrulation through regulatory myosin light-chain phosphorylation, which results in the contraction of the apical surface of ingressing cells. These findings forge new links between cell-fate specification and morphogenesis, and they represent a novel mechanism by which Wnt signaling can regulate morphogenesis.

Cell rearrangements are crucial during development. In this study, we use C. elegans gastrulation as a simple model to investigate the mechanisms of cell positioning. During C. elegans gastrulation, two endodermal precursor cells move from the ventral surface to the center of the embryo, leaving a gap between these ingressing cells and the eggshell. Six neighboring cells converge under the endodermal precursors, filling this gap. Using an in vitro system, we observed that these movements occurred consistently in the absence of the eggshell and the vitelline envelope. We found that movement of the neighbors towards each other is not dependent on chemotactic signaling between these cells. We further found that C. elegans gastrulation requires intact microfilaments, but not microtubules. The primary mechanism of microfilament-based motility does not appear to be through protrusive structures, such as lamellipodia or filopodia. Instead, our results suggest an alternative mechanism. We found that myosin activity is required for gastrulation, that the apical sides of the ingressing cells contract, and that the ingressing cells determine the direction of movement of their neighboring cells. Based on these results, we propose that ingression is driven by an actomyosin-based contraction of the apical side of the ingressing cells, which pulls neighboring cells underneath. We conclude that apical constriction can function to position blastomeres in early embryos, even before anchoring junctions form between cells.Movies available on-line

SUMMARYThe axis of asymmetric cell division is controlled to determine the future position of differentiated cells during animal development. The asymmetric localization of PAR proteins in the Drosophila neuroblast and C. elegans embryo are aligned with the axes of the embryo. However, whether extracellular or intracellular signals determine the orientation of the localization of PAR proteins remains controversial. In C. elegans, the P0 zygote and germline cells (P1, P2, and P3) undergo a series of asymmetric cell divisions. Interestingly, the axis of the P0 and P1 divisions is opposite to that of the P2 and P3 divisions. PAR-2, a ring-finger protein, and PAR-1, a kinase, relocalize to the anterior side of the P2 and P3 germline precursors at the site of contact with endodermal precursors. Using an in vitro method, we have found that the PAR-2 protein is distributed asymmetrically in the absence of extracellular signals, but the orientation of the protein localization in the P2 and P3 cells is determined by contact with endodermal precursor cells. Our mutant analyses suggest that mes-1 and src-1, which respectively encode a transmembrane protein and a tyrosine kinase, were not required to establish the asymmetric distribution of PAR-2, but were required to determine its orientation at the site of contact with the endodermal precursors. The PAR-2 localization during the asymmetric P2 and P3 divisions is controlled by extracellular signals via MES-1/SRC-1 signaling. Our findings suggest that Src functions as an evolutionarily conserved molecular link that coordinates extrinsic cues with PAR protein localization.

Abstract. 10-Hydroxycamptothecin (10-HCPT), an indole alkaloid isolated from a Chinese tree, Camptotheca acuminate, inhibits the activity of topoisomerase I and has a broad spectrum of anticancer activity in vitro and in vivo. However, its use has been limited due to its water-insolubility and toxicity with i.v. administration. The purpose of this study was to investigate the efficacy, toxicity and proper dosage of 10-HCPT as a single agent by oral administration in the treatment of human colon cancer. 10-HCPT significantly repressed the proliferation of Colo 205 cells at a relatively low concentration (5-20 nM). Flow cytometry analysis and Western blot and apoptosis assays demonstrated that low-dose 10-HCPT arrested Colo 205 cells in the G2 phase of the cell cycle and triggered apoptosis through a caspase-3-dependent pathway. Moreover, following oral administration at doses of 2.5-7.5 mg/kg/2 days, significant suppression of tumor growth by 10-HCPT was observed in mouse xenografts. No acute toxicity was observed after an oral challenge of 10-HCPT in BALB/c-nude mice every 2 days. The results of this study suggest that a relatively low dose of 10-HCPT (p.o.) is able to inhibit the growth of colon cancer, facilitating the development of a new protocol of human trials with this anticancer drug.