Investigation of lipid transporters at the peroxisome-ER interface

Abstract

Peroxisomes are small, dynamic and ubiquitous single-membrane bound subcellular compartments in eukaryotic cells. Peroxisomes are oxidative organelles with important functions in cellular lipid metabolism and redox homeostasis. In mammals, peroxisomes perform a variety of essential metabolic functions including fatty acid ɑ- and β-oxidation, and biosynthesis of ether lipids. These functions are often performed in close cooperation with other subcellular compartments including the endoplasmic reticulum (ER), mitochondria, or lipid droplets. Peroxisomes extensively communicate with the ER via membrane contact sites formed by the interaction of the peroxisomal membrane protein ACBD4/5 (acyl-CoA binding domain) with the ER-resident protein VAPA/B (vesicle-associated membrane protein). This peroxisome-ER tethering is important for peroxisome membrane dynamics, inheritance and biogenesis, as well as cooperative biosynthesis of ether-phospholipids, and disruption in peroxisome-ER contact can lead to neurodegenerative phenotypes (e.g. in ACBD5-deficient patients). Although peroxisome-ER association is crucial for supplying the lipids required for peroxisome membrane expansion, a pre-requisite for proliferation, the exact mechanisms underlying this lipid transfer are still unknown. This thesis explores the role of lipid transfer proteins at peroxisome-ER membrane contact sites in humans. Here, I characterized the vacuolar protein sorting 13 D (VPS13D) protein, and its potential role in peroxisome membrane expansion. I show that VPS13D is required for peroxisome membrane expansion and binds to the key peroxisomal membrane protein PEX14, but does not play a role in the metabolic functions of peroxisomes or peroxisome-ER tethering. Together, this highlights the importance of non-vesicular lipid transfer for peroxisomal membrane expansion. Moreover, I characterized two novel ACBD5 deficient patients’ fibroblasts to understand disease phenotypes when peroxisome-ER association is lost, revealing disrupted lipid metabolism. Finally, I contributed to the analysis of the peroxisomal protein inventory shared among vertebrates, revealing novel candidate peroxisomal proteins, and demonstrated conservation of the peroxisome-ER tether between D. rerio and humans.