The role of OCRL1 in endocytic membrane traffic

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The cells in our body are made up of internal membrane-enclosed compartments called organelles, each with their own identity and specific functions. Material is transferred between the organelles and between certain organelles and the outside of the cell via a process termed membrane traffic. One of the major membrane traffic pathways is the endocytic pathway, where material is taken up from the cell exterior via engulfment in membrane-enclosed vesicles at the cell surface. The internalised material is then sorted to various downstream compartments or recycled back to the cell surface. The endocytic pathway is fundamentally important to all cells, and when defective gives rise to numerous diseases in humans. Moreover, this pathway is exploited by pathogenic viruses and bacteria to gain entry into cells to cause infection. It is therefore important we understand how the endocytic pathway functions at the molecular level. This proposal is focussed on two enzymes that are localised to organelles within the endocytic pathway, called OCRL1 and INPP5B. These enzymes act on a class of lipids called phosphoinositides that are important for controlling various aspects of cell behaviour including endocytic membrane traffic. Mutation of OCRL1 causes two diseases in humans, called Lowe syndrome and Dent-2 disease, which are characterised by defective brain, eye and kidney function. The mechanisms by which mutation of OCRL1 brings about the pathological changes seen in Lowe and Dent patients remain unclear. We have identified a new binding partner for OCRL1 and INPP5B called IPIP27A that is essential for trafficking within the endocytic pathway. The aim of this proposal is now to determine how IPIP27A functions together with its interaction partners to regulate endocytic trafficking. Our hypothesis is that OCRL1/INPP5B and IPIP27A participate in the formation of membrane carriers on endocytic organelles to ensure trafficking occurs in a properly controlled manner. How this process occurs remains poorly understood. We plan to use a combination of experimental approaches to dissect the mechanisms involved. This will entail analysis of endocytic trafficking in tissue culture cells using various established methods including imaging of living cells in real time. Our work will provide important knowledge as to how the endocytic pathway functions in all cells, as well as reveal the mechanisms underlying Lowe syndrome, Dent disease and other disorders caused by defects in endocytic trafficking.