Non-classical functions of the endocytic protein, Epsin, in signaling regulation
Abstract
Endocytic proteins are misregulated in some developmental diseases and cancers. Among these are members of the Epsin family which are clathrin endocytic adaptors. Although endocytosis is classically linked to signal transduction due to its ability to control the presentation of membrane receptors, the Epsins have been shown in yeast to fulfill an additional endocytosis-independent role in signaling. This is due to their ability to bind and modulate the activity of GAPs (GTPase Activating Proteins). Thus, we sought to determine whether or not this Epsin function was conserved in mammalian cells. We found that the Epsins bind two GAP proteins, RalBP1 and Ocrl1. RalBP1 up-regulation is commonly associated with several types of cancers whereas Ocrl1 deficiency is the cause of a lethal developmental disease called Lowe Syndrome. We found that the Epsin family members bind RalBP1 and promote fibronectin-stimulated Arf6 and Rac1 GTPase signaling which results in enhanced cell migration and fibrosarcoma cell invasion. Additionally, the Epsin-Ocrl1 interaction was thoroughly characterized. Then, in order to determine the functional significance of Ocrl1's interaction, we first tested the ability of both Lowe Syndrome patient cells and Ocrl1 siRNA-treated fibroblasts to migrate. Results show that Lowe Syndrome and Ocrl1-depleted cells have defects in cell migration. Further investigation revealed that Ocrl1 also promotes cell spreading and macropinocytosis. An important topic of Lowe Syndrome research involves the functional redundancy between Ocrl1 and its close homolog, Inpp5b. Therefore, rescue experiments were performed in Lowe Syndrome cells and concluded that Ocrl1's role in cell migration is uniquely due to its ability to bind clathrin endocytic machinery. Collectively, this work provides evidence that the mammalian Epsins influence GTPase signaling by interacting with GAP proteins and also suggests the Epsins may play an important role in cancer and developmental diseases.
Degree
Ph.D.
Advisors
Aguilar, Purdue University.
Subject Area
Molecular biology|Cellular biology
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