The role of mVps45 in regulating GLUT4 trafficking in 3T3L1 adipocytes

Abstract

Insulin stimulates glucose transport in fat cells by inducing the movement of glucose transporters (Glucose transporter-4) from specialised storage vesicles to the plasma membrane. Insulin resistant individuals and those with Type II Diabetes exhibit impairment in the ability of insulin to stimulate glucose transport. The molecular mechanisms of glucose transporter-4 trafficking in adipocytes are an important focus in understanding the underlying etiology of this disease. Glucose transporter-4 (GLUT4) recycles between the plasma membrane and intracellular stores in the absence of insulin using a complex intracellular pathway. This involves two intracellular cycles: one is the prototypical endosomal system, the other a specialised cycle involving the trans-Golgi network and a sub-set of intracellular vesicles called GSVs (the slow cycle). Understanding the control of the entry into this second cycle is the subject of this thesis. In particular, the work in this thesis will examine the role of Syntaxin 16 and its cognate Sec1/Munc18 protein mammalian Vps45 (mVps45). The regulation of Syntaxin 16 has not been fully elucidated and understanding the role of Syntaxin 16 in SNARE complex regulation and subsequent control of GLUT4 traffic into the slow cycle requires an understanding of its cognate binding partner Sec1/Munc18 (SM) protein, mammalian Vps45 (mVps45). The absolute levels of both Syntaxin 16 and mVps45 were quantified and found to be present in 3T3-L1 adipocytes in roughly stoichiomeric amounts. IP experiments also showed the ability of mVps45 to interact with Syntaxin 16 in the absence of insulin. Using the model eukaryote Saccharomyces cerevisiae, we found that mVps45 could complement for the deletion of Vps45p. Assays for CPY secretion showed that mVps45 is able to complement for the loss of Vps45p function in the trafficking of carboxypeptidase Y (CPY). Additionally, mVps45 mutants were made that correspond to yeast mutants made previously in the lab and were tested for homology of function. Depleting 3T3-L1 adipocytes of mVps45 showed alterations in the levels of GLUT4 protein as well as the protein levels of Syntaxin 16, IRAP, and Rabenosyn. Insulin-stimulated deoxyglucose uptake was also profoundly decreased upon depletion of mVps45. Further experiments using mVps45 depleted cells show that these cells lose their sensitivity to insulin and that the loss of mVps45 in these cells causes GLUT4 to have the inability to enter the slow cycle. Taken together, these findings demonstrate that mVps45 has a role in allowing GLUT4 entry into the slow cycle