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Sphingolipid metabolism and signaling: embracing diversity

First discovered in the 1870s, sphingolipids are a fundamental class of molecules and were considered for many years to be inert structural lipids embedded in membrane bilayers. Thirty-five years ago, this all changed when the signaling functions of sphingolipids were first uncovered [1], and since then, biological functions of many members of the sphingolipid family have been identified, the metabolic network of enzymes that control sphingolipid levels has been defined, and defects in the production and breakdown of various sphingolipids have been associated with a number of pathological conditions and human diseases (reviewed in refs [2–4]). In the last decade or so, our growing understanding of sphingolipid signaling combined with advances in analytical tools and methodologies has broadened the boundaries of the sphingolipid universe. This is most clearly reflected at the molecular level by the great expansion of the number of unique sphingolipid moieties but is also evident in the rise of new scientific disciplines such as biophysics, chemistry, immunology, and structural biochemistry, among others that are becoming more prominent in the sphingolipid arena. Study of sphingolipid functions now extends beyond Saccharomyces cerevisiae and mammalian cells to encompass plants, Drosophila, C. elegans, and opportunistic pathogens such as Cryptoccous neoformans. The FEBS Letters special issue titled ‘Sphingolipid biology’ comprises 10 contributions that reflect some of this diversity in sphingolipid research to provide the reader with a fair, yet incomplete, idea of how diverse sphingolipids influence cell and organismal pathophysiology. In this editorial, we take advantage of the heads-ups provided by this special issue to briefly summarize recent emerging areas of sphingolipid research with particular emphasis on diversity in sphingolipid species and variety in approaches. Structural diversity is common across different lipid metabolic domains, but sphingolipids still represent an extreme case in this sense. While the sphingoid backbone remains the core of all sphingolipid molecules, variations in acyl chain length (both of the backbone and n-acylated fatty acids) as well as the number and position of both double bonds and hydroxyl groups can be combined with hundreds of different headgroups giving rise to thousands of unique compounds [5]. Thus, the sphingolipid chemical space ranges from simple long-chain bases to complex glycosphingolipids where the ceramide backbone is linked to glycans composed of up to 20 sugars. This structural diversity along with the bioactive nature of many of these sphingolipids poses fundamental questions about the overall role of sphingolipids in biology. Our understanding of this is still shallow and new roles for long-established sphingolipid mediators continue to be