Sphingolipid metabolism and signaling: embracing diversity.

Abstract

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 uncovered. Recent developments with reference to sphingosine 1-phosphate in cancer [[6]] and glycosphingolipids in cancer immunology [[7]] are reviewed here by Pyne and Yu, respectively. Similarly, the review by Vitner discusses sphingolipid involvement in the innate immune response of the brain, particularly in sphingolipidoses—diseases caused by mutations in sphingolipid enzymes [[8]]. One practice that may have hindered progress for many years is that sphingolipid species such as ceramide were functionally considered as one entity; it is only recently that hints of functional complexity have begun to emerge with different subspecies of the same lipid promoting distinct outcomes. An excellent example of this complexity in regulating the endoplasmic reticulum stress response is detailed in the review by Park & Park [[9]]. To disentangle this intricate problem, future sphingolipidologists will need to clarify the molecular targets of sphingolipid regulation and how such targets are impacted by these variations in sphingolipid structure. Indeed, there is early evidence of protein targets that have highly selective interactions with specific subspecies of a given sphingolipid [[10, 11]]. Further adding to this complexity is the subcellular localization as sphingolipids are synthesized and turned over across multiple cell compartments. Notably, many sphingolipids are hydrophobic and often remain within the membrane where they were produced, and there is an emerging appreciation that the biological functions of any given lipid can vary depending on its localization within the cell [[12]]. This concept of localized sphingolipid function is unfolded in the context of cilia and microvilli by Kaiser et al. [[13]]. As more complex signaling patterns for sphingolipids emerge, the research into the regulation of sphingolipid enzymes takes center stage. In this context, it is also important to look beyond traditional substrate-driven and post-translational regulatory mechanisms that are typical for metabolic enzymes. The study by Daian et al. [[14]] discussing regulation of sphingomyelin synthase 1 through its 5′ untranslated region adds to a growing body of literature detailing transcription and translational regulation of sphingolipid metabolism. The more these nuances in sphingolipid signaling increase, the more studies embracing both structural heterogeneity and spatial distribution of sphingolipids in relation to their targets will be necessary and critical to deepen our understanding of their physiological and pathological roles. The structural complexity and compartmentalization of sphingolipid calls for new approaches to systematically tackle, visualize, and characterize sphingolipids in cells and tissues which, in turn, demands combined efforts from multiple angles and fields of research. Recent work has focused on the development of new chemical sensors and tools that are better able to target specific metabolic steps of the sphingolipid pathway and to visualize in situ enzymatic activity. Similarly, the advent of targeted enzymes and click-chemistry compounds has proven effective at inducing localized increases in specific lipids [[15]]. These tools are invaluable for elucidating the function of a given metabolite and can be coupled with specific cellular localizations. This modern approach is used by Jain et al. [[16]] to dissect effects of ceramide in the mitochondria. Technological advances in analytical equipment with better sensitivity allow distinguishing among lipid species with little structural differences, whereas improved resolution of mass spectrometry imaging methodologies [[17]], among others, makes it possible to address questions related to subcellular localization, trafficking, and asymmetry, of lipids across bilayers. In the last decade or so, there has been growing interest in how sphingolipids influence the biophysical properties of membranes, which has spurred the development of probes and toolboxes for use in cell biology, biochemical, and biophysical experiments [[18]]. Some of these probes are able to track specific sphingolipid moieties and can help provide further insight into the structure–function relation of the sphingolipids as individual species or as components of a complex membrane environment. However, development of such tools is still in its infancy and alternative approaches for answering mechanistic questions are required. Some of these are based on the use of model membranes and synthetic models with features that mimic biological environments, an approach that is used by Sarmento et al. [[19]] to analyze organization of gangliosides in microdomains. Indeed, despite a lack of biological complexity, these strategies can address how particular lipid structural characteristics dictate their physico-chemical behavior in membranes. Therefore, they are commonly employed as in vitro surrogates for different model organisms, including mammals, fungi, and plants, the different sphingolipid structure and composition of which has distinct effects on membrane organization and properties. Nonetheless, achieving a mechanistic understanding of the biological implications of such differences requires combining the above-mentioned in vitro reductionistic approaches with the study of more complex (multi)cellular model organisms. The reviews by Santos et al. [[20]] and Mamode-Cassim et al. [[21]] build a strong case for the utility of fungi and plants as model systems to unveil the sphingolipid biology. Identifying the molecular mechanisms by which sphingolipids regulate signaling pathways, modulate membrane biophysical and mechanical properties, and coordinate membrane-associated events are fundamental steps toward a comprehensive understanding of the role of sphingolipids as central molecules in cell pathophysiology. Crucially, they will also provide valuable information for development of sphingolipid-based and/or sphingolipid-targeted therapeutics, further underscoring the necessity for joint efforts and multidisciplinary approaches in sphingolipid research. After 35 years of effort, the sphingolipid community has made significant progress toward establishing this family of bioactive lipids as important signaling mediators. However, as the sphingolipid universe continues to expand, earlier paradigms continue to be challenged and the once seeming black-and-white nature of sphingolipid functionality is slowly giving way to a fine tuned, orchestrated response that is dictated both by the lipid species and their subcellular localization. Although this adds an additional layer of difficulty and makes research into lipid functions more challenging and daunting, we must accept this challenge. The words of Dr. Lina Obeid—a leading member of the sphingolipid community whom we lost last year—are illuminating: ‘Don’t be discouraged because something is hard. Only good things are hard and they are worth doing’. Indeed, it is our view—as junior members of the sphingolipid field—that embracing this diversity will help us move the field forward and give us the opportunity to enhance the visibility of sphingolipid research in the international scientific community. In this spirit, we guest editors have recently established a website—sphingolipidbiology.com—as a central hub for the lipid community with the goal of providing space to present research, exchange ideas, and advertise career opportunities. In order to disseminate sphingolipid research to the wider scientific community and to counter seclusion in times of COVID-19, we have also used this platform to start a webinar series allowing researchers at all levels to present their latest work. Dedicated to Lina M. Obeid (1955–2019). Christopher J. Clarke is an assistant professor in the Department of Medicine and Cancer Center at Stony Brook University, New York. His research interests are focused on understanding how dysregulation of sphingolipids promotes cancer initiation and progression. With this knowledge, he hopes to develop sphingolipid-based therapeutics that are effective for treatment of metastatic cancer and can enhance the efficacy of existing treatments. He also hopes to define mechanisms of regulation and function of key sphingolipid enzymes relevant for cancer. Giovanni D'Angelo is Assistant Professor and Kristian Gerhard Jebsen Chair on Metabolism at the Swiss Federal Institute of Technology in Lausanne (EPFL). His main interests are understanding the meaning of compositional variability in cell membranes by studying the mechanisms by which the lipid composition is determined. Liana C. Silva is an FCT investigator at the Faculty of Pharmacy from the University of Lisboa and has a background in biochemistry and cell biology, quantitative photophysics, and molecular biophysics. Her research bridges membrane biophysics and cell biology and is focused on membrane lipids and their interplay in biological membranes. She aims to evaluate their role in membrane organization and function and to provide the molecular tools to develop improved therapeutics.