Lipid transport: molecular mechanisms and role in pathogenesis

About

Lipids are the essential building blocks of cell membranes. Many of them, with different properties, are distributed in the cell precisely to compose the membranes of organelles, such as the endoplasmic reticulum or the Golgi apparatus, as well as the plasma membrane. These membranes have their own molecular identity which is essential to ensure the localization and correct functioning of many proteins. It is, therefore, mandatory to decipher the molecular mechanisms responsible for the intracellular distribution of lipids to understand a majority of cellular events. Given that alterations in this distribution can lead to pathophysiological states, this work is also valuable because it makes it possible to define the molecular origins of various diseases.

In order to understand how the intracellular distribution of lipids is established, we are looking at how various cytosolic proteins called Lipid Transfer Proteins (LTPs) precisely and rapidly move specific lipids from one point to another in the cell. Our current goal is to understand the mode of action of different LTPs of the ORP/Osh family as well as CRAL-TRIO, PITPs, and START families to extend our knowledge of lipid transport in standard and pathological cellular contexts. We also want to characterize molecules capable of inhibiting sterol transport for anti-cancer therapies.

In terms of technology, to conduct our studies, we use sophisticated in vitro reconstitution systems with purified proteins and artificial membranes, similar to synthetic biology, with specific fluorescence-based assays (FRET). In these systems, we can measure how different LTPs transfer lipids between membranes and how their activities are coupled with lipid metabolism. We also drive cell biology investigations by confocal and super-resolution microscopy (STED) to analyze the localization of lipid transfer proteins and their ability to deliver specific lipids into organelles. We drive biochemical and structural analyses and in silico simulations to understand at the atomic level how LTPs interact with their ligands and cell membranes during a transport cycle.

Figure 2. Confocal microscopy of cultured HeLa cells. Phosphatidylserine (PS) enrichment in the cytosolic leaflet of the plasma membrane is visualized using a fluorescent PS sensor called LactC2-GFP. The presence of cholesterol in the outer leaflet of the plasma membrane is observed with the sterol sensor D4-mCherry.

Figure 3. Crystallographic structure of the Osh6 protein (in gray) in complex with a phosphatidylserine molecule (in pink). This protein contains a binding pocket to accommodate the acyl chains and polar head of this lipid. The opening of the pocket is controlled by a molecular lid (in green). The film also shows the residues constituting the binding pocket and the hydrogen bonds between these residues and key residues interacting with the lipid.