Background In the last stage of the C. elegans equivalent of epiboly, an open pocket in the epidermis is closed by marginal epidermal P/pocket cells that express and require VAB-1/Eph and PLX-2/plexin receptors for migration toward and alignment with contralateral partners at the ventral midline. Cellular mechanisms affected by these signaling proteins remain unknown. Results A cellular bridge comprising four neuron cell bodies that spans the open pocket serves as a substratum for migration of contra-lateral P cell pair P9/10 to the midline which can facilitate similar migration of neighboring P cells. This bridge is formed by a stereotypical rearrangement of five sister pairs of PLX-2 and VAB-1 expressing cells, of which three pairs serve as a scaffold for bridge assembly and two pairs form the bridge. Bridge formation requires VAB-1 kinase-dependent extension of presumptive bridge cell protrusions toward the ventral midline. An unassembled mutant bridge obstructs but does not block P cell progression toward the midline, however, cell type-specific rescue experiments show that VAB-1 or a nearly complete cytoplasmic deletion of VAB-1 expressed by scaffold and bridge cells or by P9/10 can facilitate P cell progression to the midline. MAB-20/semaphorin and VAB-1 also exhibit complex redundancies to regulate adhesion and prevent gaps between sister bridge and scaffold forming cells that would otherwise completely block P cell migration. Conclusions The Eph receptor functions to mediate cell extensions required for bridge formation, independently facilitates P cell migration to the ventral midline, and acts redundantly with PLX-2/plexin to prevent gaps between sister plexin band cells that normally serve as a substratum for P9/10 cell migration.
In this study, we demonstrate the developmental activation, in the zebrafish embryo, of a surveillance mechanism which triggers apoptosis to remove damaged cells. We determine the time course of activation of this mechanism by exposing embryos to camptothecin, an agent which specifically inhibits topoisomerase I within the DNA replication complex and which, as a consequence of this inhibition, also produces strand breaks in the genomic DNA. In response to an early (pre-gastrula) treatment with camptothecin, apoptosis is induced at a time corresponding approximately to mid-gastrula stage in controls. This apoptotic response to a block of DNA replication can also be induced by early (pre-MBT) treatment with the DNA synthesis inhibitors hydroxyurea and aphidicolin. After camptothecin treatment, a high proportion of cells in two of the embryo's three mitotic domains (the enveloping and deep cell layers), but not in the remaining domain (the yolk syncytial layer), undergoes apoptosis in a cell-autonomous fashion. The first step in this response is an arrest of the proliferation of all deep- and enveloping-layer cells. These cells continue to increase in nuclear volume and to synthesize DNA. Eventually they become apoptotic, by a stereotypic pathway which involves cell membrane blebbing, "margination" and fragmentation of nuclei, and cleavage of the genomic DNA to produce a nucleosomal ladder. Fragmentation of nuclei can be blocked by the caspase-1,4,5 inhibitor Ac-YVAD-CHO, but not by the caspase-2,3,7[, 1] inhibitor Ac-DEVD-CHO. This suggests a functional requirement for caspase-4 or caspase-5 in the apoptotic response to camptothecin. Recently, Xenopus has been shown to display a developmental activation of the capability for stress- or damaged-induced apoptosis at early gastrula stage. En masse, our experiments suggest that the apoptotic responses in zebrafish and Xenopus are fundamentally similar. Thus, as for mammals, embryos of the lower vertebrates exhibit the activation of surveillance mechanisms, early in development, to produce the selective apoptosis of damaged cells.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.