molecular and genetic analysis of the insect steroid molting hormone ecdysone in Drosophila
	William Segraves, Ph.D. Associate Dean of Yale College, Dean's Advisor on Science Education, Lecturer and Research Scientist Email: william.segraves@yale.edu Room: KBT 1100A Phone: (203) 432-8915/ (203) 432-3661

Steroid hormones are important regulators of higher eukaryotic growth, development and reproduction. These hormones act through a family of related receptors which bind to the promoters of responsive genes and activate them in response to cognate hormones. We are interested in under-standing the molecular mechanisms through which these hormones and their receptors regulate gene expression in development and reproduction.

In the fruit fly Drosophila melanogaster, the steroid insect molting hormone ecdysone is responsible for the regulation of developmental events including the dramatic metamorphosis from the worm-like larva to the winged adult. We have focused on the molecular action of ecdysone, and of a gene, E75, which is one of the primary targets of the ecdysone receptor. E75 and other early ecdysone-responsive genes are thought to be involved in the control of ecdysone-triggered regulatory hierarchies under-lying tissue and stage-specific hormone responses. Consistent with its proposed regulatory role, E75 encodes a family of DNA binding proteins capable of regulating the activity of target genes. Recently, we have obtained evidence that similar hierarchies control critical aspects of oogenesis in Drosophila and in other insects including the yellow fever mosquito Aedes aegypti. Our research is presently focused in two areas:

1) E75 function in larval development and metamorphosis. Analysis of over two dozen E75 mutations indicates that the different proteins carry out genetically separable functions at distinct developmental stages, and thus differential expression of these proteins is a potential mechanism through which stage-specific response to ecdysone could be controlled. Various classes of E75 mutations lead to a variety of phenotypes including embryonic head involution defects, larval molting defects, and aberrant metamorphosis. We are interested in extending these studies on the E75 mutants to understand in detail the cellular processes underlying these developmental disruptions and in particular the role of E75 in integrating ecdysone response with the response to other developmental signals. During larval development, E75 may be a critical mediator of Juvenile Hormone (JH)-dependent modulation of ecdysone response; E75 mutations display premature pupariation suggestive of altered JH-ecdysone integration, and studies in the moth Manduca sexta suggest that E75 may be a direct target of JH action. E75 expression patterns at the onset of metamorphosis and the phenotypes of other E75 mutations suggest that E75 may be involved in the integration of ecdysone response with signaling mechanisms responsible for the specification of specific cell fates within the eye.

2) E75 function in insect repro-duction. In the yellow fever mosquito Aedes aegypti, the progression of oogenesis from pre-vitellogenic to vitellogenic stages is controlled by ecdysone synthesized in response to a blood meal. In order to investigate the molecular mechanisms of reproductive ecdysone response in the mosquito, we have undertaken an analysis of early gene expression in vitellogenic ovary and fat body. We find that E75 is expressed in ovary and fat body in response to a blood meal, suggesting that there may be substantial similarity between repro-ductive and developmental ecdysone response hierarchies. Analysis of early gene expression in the Drosophila ovary suggests that early ecdysone responsive genes are expressed during mid oogenesis, and that they are required for progression through an ecdysone-dependent check-point analogous to that in mosquitoes. Within follicle cells, E75 is regulated both by ecdysone and the dorsal-ventral polarity system, and may be required for the integration of spatial and temporal signals in the specification of dorsal follicle cell fates. We are presently investigating the mechanisms through which E75 and other ecdysone-regulated genes are controlled within the ovary and fat body and the identity of downstream genes through which E75 and other components of ecdysone-triggered hierarchies control specific events during reproduction.

Through these and related studies, we hope to gain a unique picture of the spectrum of molecular events leading from the initial events underlying ecdysone response to its dramatic reproductive and developmental effects.

Selected Publications

Segraves, W.A. (1998) Ecdysone Response in Drosophila, in Hormones and Growth Factors in Development and Neoplasia, eds., Dixon, R.B. and Salomon, D.S., pp. 45-78.

Pierceall, W.E., Li, C., Biran, A., Miura, K., Raikhel, A.S. and Segraves, W.A. (1999) E75 expression in mosquito ovary and fat body suggests reiterative use of ecdysone-regulated hierarchies in development and reproduction. Mol. Cell. Endocrinol. 150: 73-89

Buszczak, M., Freeman, M., Carlson, J., Bender, M., Cooley, L. and Segraves, W.A. (1999) Ecdysone response genes govern egg chamber development during mid-oogenesis in Drosophila. Development 126: 4581-4589.

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