Yasushi Hiromi - US grants

The aim of this work is to study how fates of individual cells are controlled during generation of the neuronal diversity. In the previous work we performed an enhancer trap screen to search-for genes expressed in a subset of neurons or their precursors in Drosophila. Two genes/lines that is likely to act in successive steps in specification of identities of individual photoreceptor cells will be studied. These genes may have similar roles in cell specification during CNS development as well. The seven-up gene was identified from an insertion line that expresses the reporter gene in a subset of photoreceptor cells in the eye, and a subset of neuroblasts in the embryonic CNS. The seven-up gene is a member of the steroid receptor gene superfamily and is a Drosophila homologue of the human transcription factors COUP and ARP-1. seven-up mutants are embryonic lethal and have defects in several neuroblast lineages. In the eye, loss of seven-up function results in transformation of photoreceptor cells that normally express seven-up to another photoreceptor cell type. To define how the seven-up mutant phenotype is generated, cellular and molecular phenotypes will be analyzed at the time of seven-up expression. Mutant alleles will be generated that have molecular defects in specific domains of the seven-up protein and will be examined for correlation with specific phenotypes. Functional significance of the spatially restricted pattern of the receptor expression will be addressed by generating transgenic animals that express seven-up ectopically. Dominant phenotypes created by ectopic expression will be used to search for other genes in the same pathway by conducting genetic screens for suppressors of that phenotype. Spatial and temporal pattern of the seven-up ligand will be examined to analyze the significance of the ligand-receptor interaction. The H214 line expresses the lacZ reporter gene also in a subset of photoreceptor cells in the eye and a subset of CNS neurons in the embryo. Patterns of its expression in the wild type and seven-up mutant eye strongly suggest that the gene represented by the H214 line (the h2l4 gene) is regulated by the seven-up gene activity, and is responsible for a part of the seven-up mutant phenotype. Mutations in the h214 gene will be generated to address its function during ommatidial assembly in the wild type and in seven-up mutant background. The h2l4 gene will be cloned and its pattern of expression will be confirmed with RNA and antibody probes. Its promoter will be analyzed to determine the sequence element through which the seven-up product acts. The information generated for these two genes in the eye system will be used to gain insight into their functions during CNS development.