Membrane Identity

Lipid contributions to membrane identity in health and disease

The complex internal organization of the cell into distinct membrane bound organelles creates a fundamental problem for the membrane proteins that traffic through it: how to target specific membranes, and restrict their activity solely to these membranes? The inositol lipids are found exclusively in the cytosolic leaflets of organelle membranes linked by the secretory and endocytic pathways; their interconversion corresponds loosely with the flow of membranes between compartments. A popular model is therefore that inositol lipids serve principally to restrict suites of proteins to their appropriate membrane compartment. Put another way, the presence of a given inositol lipid in a particular membrane contributes to that membrane’s unique identity, and helps the appropriate proteins identify it. As membrane flows through the endocytic pathway, inositol lipid modifying enzymes alter the lipid head groups to correspond with their new cellular locale.

This model has important ramifications for a host of rare monogenic syndromes caused by mutations that inactivate inositol lipid phosphatases. These diseases include Lowe and Joubert syndromes (caused by mutations in PtdIns(4,5)P2 5-phosphatases) and Charcot-Marie-Tooth disease (myotubularin PtdIns3P/PtdIns(3,5)P2 3-phosphatases). These diseases affect a variety of organs (such as eyes, kidney and brain) but appear to be driven by failures in the membrane trafficking systems of cells in the affected tissues. A reasonable hypothesis is therefore that the cellular defects that cause these diseases emerge from accumulation of inositol lipids in inappropriate membrane compartments, leading to the accumulation of the wrong suites of effector proteins and the eventual failure of specific traffic pathways in the cell. In short, there is a breakdown of membrane identity in these cells.

A fluorescent PI(4,5)P2 biosensor detects experimentally induced accumulation of the lipid in endosomes in a living cell

We aim to test this hypothesis, taking advantage of our lab’s expertise both in probes to localize inositol lipids in living cells, and in chemical genetic approaches to manipulate the lipids in specific membrane compartments. The project entails both attempts to correct cellular deficits in cellular models of the diseases, as well as testing the key hypotheses by re-creating phenotypes in otherwise healthy cells. The approach relies heavily on molecular genetics, chemical genetics, genome editing and live-cell imaging.

Hammond lab