Membrane Traffic and Lipid Dynamics in Neuronal Function

Overview: The goal of my laboratory is to elucidate mechanisms that control the dynamics of cell membranes, with special emphasis on their role in neuronal function. My studies on membrane recycling at synapses highlighted the importance of the chemistry of bilayer lipids - phosphoinositides in particular - in the control of membrane traffic. These studies were the starting point for my long-term focus on phosphoinositides in neurons and non-neuronal cells, an area of research that we continue to pursue. These studies, in turn, made us interested in the mechanisms that control membrane lipid homeostasis. Work by our and other labs has shown that vesicle-independent lipid transport at contacts between organelles is far more important in this control than previously appreciated. The endoplasmic reticulum (ER) is the site where most membrane lipids are synthesized. Traffic of lipids from the ER to other membranes, and return of their hydrophobic catabolic products to the ER for metabolic recycling, occurs in part via vesicular carriers along the secretory and endocytic pathways. However, this traffic occurs in parallel with non-vesicular mechanisms mediated by Lipid Transport Proteins (LTPs) that extract lipids from bilayers and deliver them to other membranes while harboring them in hydrophobic cavities. Many of these proteins also function as membrane tethers, holding the ER in proximity of another membrane as they transport lipids. Transfer of lipid via LTPs at membrane contact sites operates in all neuronal compartments, as the ER reaches even the most distal branches of axons and dendrites.

Until recently, we had worked primarily on LTPs that act at ER-plasma membrane contact sites, including the Extended-Synaptotagmins and TMEM24, a protein selectively enriched in neurons. More recently we have started to work on LTPs that function at contact between the ER and either mitochondria or endosomes/lysosomes, such as VPS13 family proteins and PDZD8. VPS13 family proteins (4 in mammals) are of special interest, as mutations in each of them lead to neurodevelopmental or neurodegenerative conditions, including a Huntington-like disease (neuroacanthocytosis, mutations in VPS13A) and Parkinson’s disease (mutations in VPS13C). The special impact of these mutations on brain regions involved in the actions of drugs of addiction (the striatum and dopaminergic neurons) make this topic particularly relevant to the scope of NIDA. Further elucidating the properties, mode of action and function of these proteins are main priorities of our lab for the future. A collaboration with the laboratory of Karin Reinisch (a structural biologist, Yale Department of Cell Biology) has suggested that VPS13 family protein transfer lipids by a completely novel mechanisms (tunneling lipid from one membrane to another), making these studies of special interest.

In addition to studies of lipid dynamics, we are also continuing studies of membrane traffic at synapses. In this area we currently focus on the role of phase separation mechanisms in the assembly of membranous organelles. The recently developed concept that macromolecules, such as proteins and RNAs, can self-assemble within the cytoplasm into distinct liquid phases is having a major impact in cell biology. We have found that even membranous organelles can self-organize into liquid phase (phases in which the organelles are clustered, yet motile within the clusters) and we are investigating the role of these mechanisms in the generation and dynamics of synaptic vesicle clusters at synapses.