Regulation of Dronc Transcription by the Hippo and Ecdysone Pathways in Drosophila melanogaster

Date of Award

2022

Degree Name

Ph.D. in Biology

Department

Department of Biology

Advisor/Chair

Madhuri Kango-Singh

Abstract

Hippo pathway is an organ size regulating pathway that has implications in organogenesis, cell competition, compensatory proliferation, and regeneration. Previous studies have identified the role of impaired Hippo pathway in cancer and Alzheimer's. Down-regulation of pathway causes over proliferation by upregulation of target gene expression that promotes proliferation and prevents apoptosis. On the contrary, pathway hyper-activation causes cell death by upregulating a pro-apoptotic protein Hid and without affecting the expression of Diap-1, a target protein that prevents apoptosis. However, down-regulation of Hid fails to significantly rescue Hippo pathway mediated cell death. Work from a previous graduate student in our lab has shown that Hippo signaling regulates a key cell death gene dronc which is a homolog of the initiator caspase-9 in mammals. When Hippo levels are high, Yki is sequestered in the cytoplasm and dronc levels are high and cell death is the result. In contrast to that when Hippo is downregulated, Yki is free to move to the nucleus and cause changes in gene expression and dronc is also downregulated and the result is proliferation of cells. But the phenomenon between Yki and dronc are poorly studied. The aim of my work was to thus identify how Yki that interacts with Hippo pathway to regulate cell death via dronc regulation. Dronc is also a common target to the Ecdysone signaling which is an important steroid hormone in insects that allows spatio-temporal of gene expression to regulate events of cell death and growth during molting and metamorphosis. But the relationship between Hippo Signaling and Ecdysone signaling to regulate a common target such as dronc is underexplored. To attain this goal, we tested for genetic interaction between known Ecr pathway components and the Hippo pathway components to determine if there is any genetic epistasis between the components. Our experiments led to the identification of a feedback loop in which the downstream nuclear effectors of the Hippo pathway, Yki and Sd regulate expression of EcR and in turn EcR regulates Yki and Sd. And this feedback seems to be important for dronc regulation. Using genetic and biochemical techniques we further confirmed that there are Sd binding sites on the 1.4kb region of the dronc promoter and these binding sites are important for maintaining optimum levels of dronc in the wing imaginal discs of Drosophila. Glioma is an aggressive form of adult brain tumor with poor prognosis and low survival outcomes. The recurrence of glioma after radiation has been associated with presence of glioma stem-like cells. Our collaborator, Dr. Nakano's group observed CD109 (Drosophila Tep1) protein is expressed by the MES glioma like stem cells after radiation treatment of the proneural CD144 expressing tumor cells which helps in providing radioresistance with increased clonogenecity. Using mammalian and Drosophila models, we confirmed an evolutionarily conserved role of CD109 in glioma progression. Exposure of glioma to ionizing radiation led to transcription of CD109 by activated NF-kB and silencing of CD109 repressed transcription of TAZ. Downregulation of Tep1 in the Drosophila glioma model showed reduction in glioma size, proliferation and reduced Yki expression. Overall, our research helped to unravel the intricate interactions between keys signaling pathways that promote tumor progression in an in vivo model that normally do not exist in normal cells through normal development.

Keywords

Department of Biology, Dronc, Hippo Pathway, Ecdysone Signaling, Neuroblast Stem Cells, Glioblaatoma, Tep1, Yorkie, Scalloped, Cell death, Cell proliferation

Rights Statement

Copyright © 2022, author.

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