Composition and Function of Membrane Contact Sites in Differentiation and Disease

ABSTRACT Eukaryotic cells are compartmentalized into membrane-bound organelles to spatially separate incompatible biochemical processes. However, organelles must co-ordinate their activities so that cells can respond to changing developmental or environmental conditions. One mechanism of coordination is via membrane contact sites (MCSs), sites of close apposition between organelles used for the exchange of ions, lipids, and proteins. Despite being implicated in multiple metabolic and neurodegenerative diseases, the function and composition of many MCSs is not well understood. The proposed research program will continue to explore the function and composition of MCSs via two related themes. Because different cell types have diverse morphologies and physiological functions, we hypothesize that organelle structure and communication must remodel throughout differentiation of stem cells. In theme A, we will use multispectral imaging of organelles followed up with mechanistic studies to elucidate how and why MCSs rewire during differentiation of stem cells into neurons or skeletal muscle. This will identify common themes in organelle and MCS reorganization during differentiation, as well as features unique to each cell type. In theme B, we will focus on lipid droplet (LD)- organelle MCSs. LDs are lipid storage organelles with a unique structure consisting of a neutral lipid core surrounded by a phospholipid monolayer. The unique monolayer membrane means that LDs are not connected to vesicular trafficking pathways, so MCSs are the only way these organelles can exchange material with other organelles. We will use spatial proteomics to identify proteins at LD contacts with other organelles including mitochondria, the endoplasmic reticulum (ER), and the plasma membrane. We will then test the roles of LD-organelle MCS proteins in cellular functions including lipid and protein trafficking. These studies will yield important insights into how cells reorganize to support diverse cellular physiologies during normal development. In addition, our work will elucidate how defects in organelle communication contribute to conditions including metabolic, neurodevelopmental, and neurodegenerative diseases.