Award Date

5-2010

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Biological Sciences

Department

Biological Science

First Committee Member

Allen Gibbs, Chair

Second Committee Member

Deborah Hoshizaki

Third Committee Member

Andrew Andres

Fourth Committee Member

Jeffrey Shen

Fifth Committee Member

Craig Woodard

Graduate Faculty Representative

Brian Spangelo

Number of Pages

134

Abstract

Holometabolous insects undergo an astonishing transition during their development. During metamorphosis, the larva dramatically changes form and becomes an adult fly. During this process obsolete larval tissues must be eliminated, while tissues required for further development are retained and often remodeled to meet the needs of the adult fly. Tissue remodeling is characterized by morphological changes of the cells in a tissue mass. In many cases, remodeling is characterized by dissociation of the tissue mass, releasing cells to move freely around the body cavity. This process is also common in wound healing and is a key feature of human disease processes such as metastasis and airway destruction in asthmatics. The detachment of remodeled cells requires proteases that can break down the extracellular matrix, which is responsible for the integrity of the tissue. The larval fat body of Drosophila is an indispensable tissue required to fuel animal development, thus this tissue is retained and remodeled during the transition from a larva to an adult. In this dissertation I identify the functions of two important proteins in the signaling cascade responsible for the remodeling of the fat body, and I propose a model for the role of this cascade in the fat body for animal survival during metamorphosis. I performed a detailed characterization of fat-body tissue remodeling and identified three distinct stages associated with remodeling (Nelliot et al., 2006). Using several genetic techniques, I show that the hemocytes (Drosophila blood cells) are not required for fat body remodeling and that the process of fat body remodeling is tissue autonomous. I then outline a role for the 20-hydroxyecdysone (20E) signaling cascade in fat body tissue remodeling. Through expression of dominant negative forms of the 20E receptor (EcR) and mosaic analysis I have determined that signaling through the EcR and expression of the competence factor βftz-f1 are both required for fat body remodeling. I have also identified the 20E signaling target gene Matrix Metalloproteinase 2 (MMP2) as the protease required for remodeling of fat cells during metamorphosis. In addition the role of MMP2 in fat body remodeling, I show that proper expression of MMP2 is required for animal survival. Also, through mutant analysis, I show that the other Drosophila Matrix Metalloproteinase, MMP1, is not involved in fat body remodeling. However, I do demonstrate a fat body specific role for MMP1 in the process of head eversion.

Overall, these results uncover another potential role for MMP2 in the fat body during metamorphosis. My experiments show that proper regulation of MMP2 expression in the fat body is required for animal survival. In an attempt to explain the importance of MMP2 in the fat body, I propose a model where 20E signaling in the fat body modulates insulin signaling via its induction of MMP2 expression. Matrix metalloproteinases are known to cleave Insulin-like Growth Factor Binding Proteins (IGF-BPs) and thus have a regulatory effect on insulin-like growth factor signaling in mammals (Fowlkes et al., 1994). In Drosophila, IGF-BPs are involved in the protection of the Drosophila insulin-like peptides (DILPS), the ligands for the Insulin Receptor (Arguier et al., 2008; Honegger et al., 2008). Thus, the binding of IGF-BPs to DILPS modulates insulin signaling. I propose that MMP2 expression is required in the fat body to modulate insulin signaling during metamorphosis through the cleavage of IGF-BPs. With this model, MMP2 expression would connect the role of 20E signaling in the fat body to the modulation of insulin signaling during metamorphosis. In addition to my work on the regulation of fat body remodeling, I also explored the role of fat body 20E signaling in pupal metabolism. The fat body is the central tissue involved in the storage of nutrients and the sensing of nutrient availability. In this dissertation I show that 20E signaling in the fat body is required for animals survival during metamorphosis (Cherbas et al., 2003). I hypothesized that 20E signaling in the fat body is a critical factor in metabolic control of pupal development. Therefore I tested the role of 20E signaling in regulation of metabolic rate, as well as the acquisition and utilization of energy stores. Contrary to my hypothesis, I demonstrate that 20E signaling in the fat body does not affect pupal metabolism or animal's ability to attain proper size at pupariation. These data suggest that 20E signaling in the fat body is not required for proper metabolic function.

Keywords

Drosophila; Ecdysone; Adipose tissues; Matrix Metalloproteinase 2 (MMP2); Tissue remodeling; Fruit-flies; Morphogenesis; Genetic regulation

Disciplines

Genetics | Molecular Genetics

Language

English


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