Monitoring Sphingomyelin Biosynthesis at Nanoscale Resolution by Expansion Microscopy

Sphingomyelin is the most abundant sphingolipid in mammalian cells and is synthesized by two isoenzymes, sphingomyelin synthase 1 and 2, located in the Golgi and in the plasma membrane. Abnormal sphingomyelin synthesis is associated with infections and diseases such as diabetes and cancer. Measuring cellular sphingomyelin synthase activity fosters our understanding of how these enzymes are involved in pathological processes and can be crucial for the identification of therapeutic compounds modifying sphingomyelin biosynthesis. We have developed a novel fluorometric assay that enables microscopic detection of cellular sphingomyelin synthase activity. We show that sphingomyelin synthases use propargyl choline and -NH2-{omega}-N3-C6-ceramide to generate trifunctional sphingomyelin, a lipid derivative detectable with spatial resolution via Forster resonance energy transfer. By combining this assay with expansion microscopy, a super-resolution imaging technique, we measured the distribution of de novo synthesized trifunctional sphingomyelin in cells at nanoscale resolution, thereby directly demonstrating sphingomyelin biosynthesis at the Golgi and the plasma membrane. By monitoring sphingomyelin biosynthesis and degradation in cells infected with the obligate intracellular pathogen Chlamydia trachomatis, we dissected the complex sphingolipid metabolization of these bacteria with unprecedented resolution. By correlating spatial metabolic information with lipidomics, we provide a powerful tool for investigating cellular sphingomyelin metabolism.