Regulation of Sphingomyelin Synthesis by Cholesterol.

The most abundant sphingolipid of the plasma membrane (PM), sphingomyelin (SM), binds to cholesterol in a manner dependent on the structure of fatty acids in the SM, and it is a major determinant of the cholesterol content of the PM. Synthesis of SM in the Golgi apparatus produces an mol-equivalent of diacylglycerol (DAG), a signaling lipid implicated in the fission of secretory vesicles from the Golgi. We speculate that coupled synthesis of SM and DAG in the terminal compartments of the Golgi results in fission of secretory vesicles enriched in SM and integral membrane proteins that favor sphingolipid and cholesterol-rich membrane. Two classes of SMs are synthesized in the Golgi apparatus that are distinguished by the length of the fatty acyl chain on the ceramide backbone: long chain (LC; ≤C20), and very long chain (VLC; >C20) SMs. Whereas the precursor to LC-SMs is supplied to the Golgi via non-vesicular transport of LC-ceramide from the ER to the Golgi by ceramide transport protein (CERT), it is speculated that VLC-ceramide is supplied to the Golgi via bulk membrane trafficking pathways. From the Golgi, LC-SM and VLC-SM are trafficked to the PM in SM-rich secretory vesicles that also contain a select cohort of secreted proteins (Deng et al., 2018). Cholesterol extraction increases sphingomyelin synthesis (Perry and Ridgway, 2006), however, the two classes of SMs are differentially regulated; we discovered that only synthesis of very long chain (>C20) SMs was increased. Furthermore, cholesterol loading reduced the synthesis of VLC-SM, only. We then asked, what cellular components regulate VLC-SM synthesis and how do they influence the cholesterol content of the PM and other organelles. Synthesis of VLC-SM was inhibited by interfering with COPII-mediated export from the ER and by depletion of ER-localized ceramide synthase 2, which produces VLC-ceramide. Reduced VLC-SM synthesis resulted in the depletion of cholesterol from the PM inner leaflet and a concomitant accumulation in the cytoplasmic leaflets of the membranes of lysosomes and other intracellular organelles. Increased synthesis of VLC-SMs by cholesterol depletion resulted in increased the level of diacylglycerol (DAG) in the Golgi and the appearance in cytoplasmic vesicles containing TGN46 which is secreted in SM-rich secretory vesicles. The results suggest that VLC-SM synthesis is regulated by cholesterol content on the PM and cholesterol depletion-induced synthesis of VLC-SM enhances SM-specific secretory pathway from the Golgi to the PM.