Epidermal enclosure is an example of an epiboly process that involves spreading of the epidermal epithelium over substrate cells. The substrate for epidermal enclosure is formed predominantly by ventral neurons and support cells. Disruption of several signaling pathways (bidirectional Eph signaling, PTP-3, SAX-3, KAL-1/HSPGs) leads to aberrant development of the substrate and consequent defects in epidermal enclosure. The primary defect in these mutants arises in movement of ventral neuroblasts (VNBs) to the ventral midline, a process that normally closes a ventral cleft formed in gastrulation. The cellular forces driving VNB movement are not known. To address how VNBs move, we are using a combination of imaging, laser ablation and pharmacology. We will describe progress in our analysis of the basis of neuroblast migrations.
Elimination of any one of the VNB pathways results in incompletely penetrant defects in development whereas loss of two or more pathways confers synergistic defects, indicating a high level of genetic redundancy in the control of VNB movement. We have exploited this synergism by using RNA interference screens for enhancers of weak morphogenetic phenotypes. Such enhancers may define additional pathways involved in VNB movement, or additional components of the known pathways. We are especially interested in genes that might function in the ephrin reverse signaling pathway that is required in VNB migration, as little is known about how GPI-linked ephrins promote reverse signals. From an RNAi screen of 1/3 of the genome in an
efn-4(wk)
rrf-3 background we identified one strong unexpected enhancer and a large number of weaker enhancers. We will describe our characterization of these enhancers.
Studies of epiboly processes in other organisms suggest that substrate contractility plays an active role in promoting the movement of the overlying epithelium. To test if such coordinated movements may be important in C. elegans we have begun dual imaging of epidermis and substrate during enclosure.