Lipid transfer proteins at membrane contact sites between the endoplasmic reticulum and mitochondria
Date of Award
Fall 2022
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Cell Biology
First Advisor
De Camilli, Pietro
Abstract
The Endoplasmic Reticulum (ER) is a subcellular compartment distributed throughout the entire cytosol which plays a variety of functions in cell physiology, including synthesis of most membrane lipids. From the ER, lipids can be transported to other membranous organelles. This process can occur via two mechanisms: vesicular trafficking, in which lipids are transported as part of the membrane of vesicular carriers, and protein-mediated lipid transfer, in which lipids are carried between two membranes within the hydrophobic pocket of a protein. Many of these proteins act at membrane contact sites, where the short distance between two organelles enhances the efficiency and specificity of lipid transfer proteins (LTPs). LTPs can function between two membranes as shuttles, exchanging small amounts of lipids, or as bridges, allowing bulk transfer of lipids. Given that mitochondria are not connected to the vesicular pathway, its contacts with the ER play key functions in cell physiology and lipid homeostasis. However, the mechanisms of lipid transfer between the ER and mitochondria in metazoans are still poorly understood. In this thesis work, I have studied mammalian lipid transfer proteins localized to ER-mitochondria contact sites. In yeast cells, the ERMES protein complex and the Vps13 protein are known to mediate lipid transfer between the ER and mitochondria. In mammalian cells, it had been suggested that the ER-anchored and shuttle-like LTP PDZD8 localizes to ER-mitochondria contacts. However, during my thesis work I found that PDZD8 is mostly enriched at interfaces between ER and endolysosomes, where it is an effector of the GTPase Rab7. Further work by other groups have reinforced this idea by finding additional interactions of PDZD8 with another ER-endolysosome contact protein, Protrudin, where they play a role together in endosomal maturation. Given that proteins of the ERMES complex do not have phylogenetic orthologs in mammals, with the exception of the mitochondrial GTPase Gem1, we decided to focus on mammalian Vps13 homologs, the proteins VPS13A, B, C and D, which are all implicated in neurological diseases. I have found that VPS13D interacts with the mammalian ortholog of Gem1, Miro, in the mitochondrial and peroxisomal membrane to mediate the bridging of these organelles to the ER, where VPS13D binds VAP through an unconventional FFAT motif. Miro had been mostly implicated in mitochondrial trafficking, so my work revealed a so far missing link between the function of Gem1 in fungi and Miro in metazoans related to the control of lipid delivery to mitochondria. VPS13D is implicated in a form of ataxia, is essential for human cells and is important to maintain proper mitochondrial function and peroxisome biogenesis, underscoring its importance at lipid transfer at these interfaces. A related protein, VPS13A, is implicated in a neuroacanthocytosis syndrome and is also found at ER-mitochondria contacts, but loss-of-function of this protein does not lead to major defects in mitochondrial function. I have found that VPS13A can additionally localize to contacts between the ER and plasma membrane (PM), where it binds to the scramblase XK, also implicated in a neuroacanthocytosis syndrome with high similarity to VPS13A-mediated disease. My results suggest that the disease associated function of VPS13A is at these contact sites, where it is coupled with PM lipid scrambling by XK. In conclusion, the work presented here suggests that VPS13D plays a key role at ER-mitochondria contacts whereas PDZD8 and VPS13A does so at other ER contacts, with endolysosomes and with the plasma membrane, respectively. Future studies should address the mechanisms associated to the pathogenesis caused by mutations in VPS13A and VPS13D, which could be related to PM scrambling and mitochondrial function, respectively. Moreover, my thesis work has contributed to the characterization of VPS13 proteins as a novel eukaryotic mechanism for bulk transfer of lipids at ER-contact sites, previously reported to occur only in prokaryotic cells.
Recommended Citation
Samander, Andres Guillen, "Lipid transfer proteins at membrane contact sites between the endoplasmic reticulum and mitochondria" (2022). Yale Graduate School of Arts and Sciences Dissertations. 756.
https://elischolar.library.yale.edu/gsas_dissertations/756
