The ER-Mitochondria Interface and Its Role in Phospholipid Homeostasis

• Applicant: Dr. Till Stephan
• Subject Area: Cell Biology
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 560302783

The mitochondrial inner membrane, the site of oxidative phosphorylation, has a distinct lipid composition, characterized by the enrichment of phosphatidylethanolamine and cardiolipin. Synthesis of these and many other phospholipids requires interplay between mitochondria and the endoplasmic reticulum (ER), the central hub for lipid biosynthesis in eukaryotes. Mitochondria utilize phosphatidylserine from the ER to synthesize the bulk of cellular phosphatidylethanolamine, which can be transported back to the ER for the synthesis of several other phospholipids. Similarly, mitochondria import phosphatidic acid from the ER for the synthesis of cardiolipin, underlining the tight functional interplay between the two organelles in lipid metabolism. The exchange of lipids requires the formation of ER-mitochondria contact sites (ERMCSs), which bring the two organelle membranes into close proximity. Despite their critical function, the protein composition, structure, and spatiotemporal organization of ERMCSs in higher eukaryotes have remained enigmatic. Phospholipids such as phosphatidylethanolamine and cardiolipin are key for maintaining mitochondrial ultrastructure and respiration, suggesting that lipid fluxes mediated by ERMCSs could serve as key regulators of mitochondrial function. However, how the machinery for the synthesis and exchange of phospholipids adapts lipid fluxes between both organelles to different cellular conditions and how this affects mitochondrial activity remains elusive. It also remains unclear where along the continuous ER phospholipids are produced. It was long believed that ER “sheets” and “tubules” represent the rough ER for protein synthesis and smooth ER for lipid synthesis, respectively. Nevertheless, nanoscale resolution imaging revealed numerous previously overlooked morphological subdomains within the ER. How these diverse structures participate in the production of lipids and their transport via ERMCSs is unknown. In this proposal, we will address these gaps in the ER-mitochondria axis of phospholipid homeostasis by investigating its spatiotemporal orchestration in situ. By integrating advanced fluorescence imaging into a multi-level approach, we will investigate the protein composition, structure, and dynamics of ERMCSs, how they influence phospholipid homeostasis and how lipid fluxes via ERMCSs control mitochondrial form and function. Finally, we will clarify whether the ER contains subdomains for the synthesis of phospholipids and if ERMCSs form in their vicinity.

• DFG Programme: Emmy Noether Independent Junior Research Groups