Date Available

6-29-2016

Year of Publication

2014

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Arts and Sciences

Department/School/Program

Biology

First Advisor

Dr. Douglas Harrison

Abstract

The development of multi-cellular organisms requires extensive cell-cell communication to coordinate cell functions. However, only a handful of signaling pathways have emerged to mediate all the intercellular communications; therefore, each of them is under an array of regulations to achieve signaling specificity and diversity. One such signaling pathway is the Janus Kinase/ Signal Transducer and Activator of Transcription (JAK/STAT) pathway, which is the primary signaling cascade responding to a variety of cytokines and growth factors in mammals and involved in many developmental processes. This signaling pathway is highly conserved between mammals and Drosophila, but the Drosophila JAK/STAT pathway possesses only three ligands: Unpaired (Upd), Upd2 and Upd3. Co-localized expression patterns of the ligands at several developmental stages raise the possibility that they physically interact. This work was aimed at testing the protein-protein interactions between Upd-family ligands and exploring possible outcomes of ligand oligomerization.

Physical interactions between Upd-family ligands were tested using a Bimolecular Fluorescence Complementation (BiFC) assay. The data suggested that homotypic interactions of Upd2 and Upd3 were stronger than their respective heterotypic interactions with Upd, and the homotypic interaction between Upd molecules was the weakest. In addition, the homotypic interaction of Upd3 was confirmed using yeast two-hybrid interaction assays. To identify protein domains critical for Upd3/Upd3 interaction, a series of poly-alanine substitutions were made to target the 6 conserved domains of Upd3. All 6 substitutions altered the strength of Upd3/Upd3 interaction and drastically reduced Upd3-induced JAK signaling activity. In addition, poly-alanine substitutions of some domains also affected Upd3 extracellular localization or protein accumulation.

Potential outcomes of interactions between Upd-family ligands were tested both in vitro and in vivo. The interaction between Upd and Upd3 did not significantly change the level of JAK signaling activity. However, loss of Upd3 restricted the distribution of Upd in egg chambers and consequently altered the follicle cell composition. Therefore, Upd/Upd3 interaction is likely to affect the range rather than the intensity of JAK signaling in egg chambers. In summary, this study suggested the possibility of ligand oligomerization as a mechanism for regulating signaling pathways in order to achieve signaling specificity and diversity during development.

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