Dr. Suihan Feng (Riezman lab, UNIGE) presents his recent work on lipid functions and metabolism at high spatiotemporal resolution with emphasis on the difficult biochemical aspects behind it. Dr. Takeshi Harayama (Riezman lab, UNIGE) comments with a few lines to explain the approach used to solve the question. They meet through chemical biology.
Perspective by Dr. Takeshi Harayama
Lipids are the major components of cellular membranes, which delineate most organelles. It is well established that different organellar membranes have distinct lipid composition, which is critical for their functions. It is also suggested that lipid composition could differ even within a single organelle, generating sub-organellar domains. This compartment-specific composition is the result of local lipid metabolism and transport. However, it remained difficult to study lipid functions and metabolism with subcellular resolution.
To overcome this, we developed chemical biology tools to deliver lipids in specific compartments with high temporal resolution. This was achieved by synthesizing photo-caged lipids, which are lipid precursors protected by photolabile groups and can be released upon light illumination. We chemically modified the photolabile group by adding organelle tags in order to locate the caged lipids to desired compartments, such as lysosomes and mitochondria. Using these tools, we could deliver caged lipids in specific places, release them with a flash of UV, and follow their metabolism using mass spectrometry. We used this strategy to study local metabolism of sphingosine, which is the precursor of membrane lipids called ceramides (and their downstream metabolites) and the signaling lipid sphingosine 1-phosphate. We combined this approach with CRISPR-Cas9-based genetics to dissect the enzymes responsible for local lipid metabolism.
Through this research, we found that local production of sphingosine 1-phosphate is drastically different between organelles. In addition, when we monitored the conversion of sphingosine into ceramides happening in the endoplasmic reticulum, we found that ceramides with different fatty acids are produced, depending on where sphingosine comes from. Although it was thought that the fatty acids in ceramides depend on the expression levels of ceramide synthases, our results showed that the origin of sphingosine is also important, thereby revealing metabolic differences in a sub-organellar scale.
This research revealed novel aspects of lipid metabolism with unprecedented spatiotemporal resolution. Similar approaches could be used to study local metabolism and transport for broader targets, which is not limited to lipid research.
Suihan Feng, Takeshi Harayama, Dalu Chang, J. Thomas Hannich, Nicolas Winssinger, Howard Riezman, “Lysosome targeted photoactivation reveals local sphingosine metabolism signature“, Chem. Sci., 10, 2253-2258, 2019.