Molecular Mechanisms of Endocytosis: Trafficking and Functional Requirements for the Transferrin Receptor, Small Interfering RNAs and Dopamine Transporter: A Dissertation

Abstract

Endocytosis is an essential function of eukaryotic cells, providing crucial nutrients and playing key roles in interactions of the plasma membrane with the environment. The classical view of the endocytic pathway, where vesicles from the plasma membrane fuse with a homogenous population of early endosomes from which cargo is sorted, has recently been challenged by the finding of multiple subpopulations of endosomes. These subpopulations vary in their content of phosphatidylinositol 3- phosphate (PI3P) and Rab binding proteins. The role of these endosomal subpopulations is unclear, as is the role of multiple PI3P effectors, which are ubiquitously expressed and highly conserved. One possibility is that the different subpopulations represent stages in the maturation of the endocytic pathway. Alternatively, endosome subpopulations may be specialized for different functions, such as preferential trafficking of specific endocytosed cargo. To determine whether specific receptors are targeted to distinct populations of endosomes, we have built a platform for total internal reflection fluorescence (TIRF) microscopy coupled with structured illumination capabilities named TESM (TIRF Epifluorescence Structured light Microscope.) In this study, TESM, along with standard biochemical and molecular biological tools, was used to analyze the dynamic distribution of two highly conserved Rab5 and PI3P effectors, EEA1 and Rabenosyn-5, and systematically study the trafficking of transferrin. Rabenosyn-5 is necessary for proper expression of the transferrin receptor as well as internalization and recycling of transferrin-transferrin receptor complexes. Results of combining TIRF with structured light Epifluorescence (SLE) indicate that the endogenous populations of EEA1 and Rabenoysn-5 are both distinct and partially overlapping. The application of antisense oligonucleotides as potential therapeutic agents requires effective methods for their delivery to the cytoplasm of target cells. In collaboration with RXi Pharmaceuticals we show the efficient cellular uptake of the antisense oligonucleotide sd-rxRNA® in the absence of delivery vehicle or protein carrier. In this study TIRF, SLE, and biochemical approaches were utilized to determine whether sd-rxRNA traffics and functions along specific endosomal pathways. Sd-rxRNA was found to traffic along the degradative pathway and require EEA1 to functionally silence its target. These new findings will help define the cellular pathways involved in RNA silencing. Neurotransmitter reuptake and reuse by neurotransmitter transport proteins is fundamental to transmitter homeostasis and synaptic signaling. In order to understand how trafficking regulates transporters in the brain and how this system may be disregulated in monoamine-related pathologies, the transporter internalization signals and their molecular partners must be defined. We utilized a yeast two-hybrid system to identify proteins that interact with the dopamine transporter (DAT) endocytic signal. The small, membrane associated, GTPase Rin was determined to specifically and functionally interact with the DAT endocytic signal, regulating constitutive and protein kinase C (PKC) – stimulated DAT endocytosis. The results presented in this study provide new insights into functions and components of endocytosis and enhance the understanding of endocytic organization.