In the C. elegans embryo, the membrane receptor, GLP-1, is localized to anterior blastomeres where it mediates cell interactions that control anterior fates. GLP-1 localization is achieved by translational repression of
glp-1 mRNA in oocytes and posterior cells of the embryo. This regulation resembles the antero-posterior control of hunchback mRNA in Drosophila, and could underlie a fundamental system for generating polarity in the early embryo. Are the cis-acting signals that spatially restrict
glp-1 translation functionally analogous to elements in hunchback mRNA from flies? A 61 nt region of the
glp-1 3' untranslated region (3' UTR) is necessary for translational repression of a lacZ reporter mRNA in posterior blastomeres. This 61 nt region contains three motifs similar to the nanos response elements (NRE) that control hunchback translation in flies. To determine the relevance of the NRE-like motifs, we have further examined sequence requirements. A 39 nt deletion that removes all three NRE-like motifs and intervening sequences prevented posterior repression, whereas removal of a neighboring 21 nt had no effect. In addition, replacement of the intervening sequences with random sequence of the same base composition had no effect on regulation. Together, these results suggest that the NRE-like motifs may indeed be necessary for spatial restriction of
glp-1 translation in the embryo. However, hunchback NREs fail to replace the
glp-1 spatial control element in worm embryos, and the
glp-1 elements fail to replace hunchback NREs in fly embryos (D. Curtis and R. Lehmann, personal comm.). We are making point mutations in the NRE-like motifs of
glp-1, similar to mutations that eliminate hunchbback regulation in flies, to directly test their significance. What is the function of localizing
glp-1 translation during embryogenesis? We are examining the effects of ectopic GLP-1 expression on embryonic development following injection of full length
glp-1 mRNA. Preliminary experiments suggest that
glp-1 mRNA lacking translational control elements in the 3' UTR induces embryonic lethality in wild type embryos. Dead embryos contain many differentiated cell types but fail to undergo proper morphogenesis. In contrast, wild type
glp-1 mRNA has little or no impact on wild type embryos, although it can rescue the embryonic defects of a
glp-1(lf) mutant. Thus, derepression of
glp-1 translation may lead to specific lineage alterations in the embryo. We are currently testing whether lethality results from ectopic expression of GLP-1 in posterior cells, over-expression in anterior cells, or both. In addition we plan to determine the lineage defects.