We are interested in how cell polarity is controlled during metazoan development. Our approach is to identify and study genes involved in the control of cell polarity by identifying mutations that disrupt the polarities of individual cells. Mutations in one such gene, the C. elegans gene
lin-44, cause the polarity of certain cells in the tail to be reversed with respect to the body axis (Herman and Horvitz, 1994). Previous work demonstrated that
lin-44 encodes a WNT signaling protein that is made by the skin cells at the tip of the developing tail and functions to specify the polarity of more anterior tail cells (Herman et al., 1995). WNT proteins act as short-range signaling molecules in many different species. We want to learn how this signal from the tail skin cells influences the polarities of the receiving cells. One result of the reversal of T cell polarity is that the phasmids of
lin-44 animals are defective. The phasmids are sensory structures in the tail that consist of two neurons, a sheath cell and two socket cells which provide an opening to the environment through which fluorescent dyes can enter and fill the neurons. The reversal of T cell polarity in
lin-44 mutants causes the phasmid socket cells to be misplaced, which blocks dye filling of the phasmids neurons. Thus phasmid dye-filling provides a simple assay for T cell polarity. New genes that may interact with
lin-44 have been identified in a preliminary screen by looking for additional mutations that result in a phasmid dye-filling defect. We have analyzed one of these, the
egl-27 gene, in some detail (see abstract by Herman et al.) Mutations in
egl-27 also cause defects in the cell migrations of the HSNs and descendants of the QL neuroblast. The direction of migration of the affected cells in controlled by another WNT signal encoded by the
egl-20 gene. Thus mutations in
egl-27 cause defects similar to those observed in two different Wnt mutants, suggesting that EGL-27 may function in a common WNT signaling pathway. Database searches have revealed that the amino-terminal half of the putative
egl-27 protein is similar to the metastasis-associated factor Mta1, whose expression is elevated in rat and human metastasizing mammary adenocarcinoma cell lines. This suggests that this factor may be involved in a common mechanism used to interpret WNT signals. We are continuing our analysis of
egl-27 by investigating the effects of over-expression of
egl-27. We have placed the composite
egl-27 cDNA clone under control of the
hsp-16 promoter in transgenic animals. We first determined that the composite
egl-27 cDNA clone encoded functional EGL-27 be demonstrating that hs-
egl-27 could rescue the polarity defects of
egl-27 mutants after heat shock. A control construct which places a stop codon after residue 210 of the 1129 amino acid EGL-27 protein does not rescue, however. We are now using these constructs to investigate the phenotypic effects of
egl-27 overexpression. We are also continuing the screen for phasmid dye-filling defective mutations to identify additional genes that function in the control of T cell polarity. To date, we have screened approximately 26,000 additional haploid genomes and have identified six new polarity defective mutations. Two of these are additional alleles of
lin-44, and the others remain uncharacterized. We will report on the progress of this screen.