Ins and outs of the bioactive sphingolipid metabolite sphingosine-1-phosphate

PROJECT SUMMARY AND RELEVANCE The sphingolipid metabolites, sphingosine-1-phosphate (S1P) and its precursors sphingosine and ceramides, have emerged as pleiotropic signaling molecules that regulate diverse cellular responses. Previously, we have shown that in response to numerous stimuli, S1P produced intracellularly by two sphingosine kinases, SPHK1 and SPHK2, is exported out of cells via a specific transporter Spinster homolog 2 (SPNS2) to activate its receptors S1PRs. While this process that we coined “inside-out signaling by S1P” is responsible for many of its known functions, it has long been suspected that SPHK and the sphingolipids it regulates, S1P, sphingosine, and ceramide, also have intracellular functions. This is an important gap that our research and the MIRA proposal are aimed at filling as lower organisms express the evolutionary conserved sphingolipid metabolic enzymes and although sphingolipid metabolites regulate important biological functions in these organisms, they do not have S1PRs. Moreover, these enzymes are present in distinct subcellular compartments, suggesting that the location(s) where sphingolipid metabolites are produced dictates their functions. The MIRA proposal is focused on the roles of these sphingolipid metabolites in governing membrane contact sites (MCS) between the ER network and late endocytic organelles versus the plasma membrane to control the movement of cholesterol and sphingolipids between distinct cell membranes and their metabolism. Our research will also address the involvement of these sphingolipid metabolites in establishment of lysosome-mitochondria MCS and consequence to mitochondrial function and bioenergetics and define how they regulate localization of GRAMD1s family, evolutionarily conserved ER-anchored cholesterol transfer proteins. Another goal of the proposed research is to understand the functional transport mechanism of SPNS2, which remains controversial, and could have important ramifications as SPNS2 has been linked to metabolism, cancer metastasis, and auditory, inflammatory, and immune disorders. The new conceptual groundwork in this proposal will alter our understanding of sphingolipid metabolites signaling and will reveal their ancient but understudied intracellular roles in regulation of membrane dynamics and contact sites as sensors that integrate and coordinate sphingolipid and cholesterol metabolism, and potentially glucose homeostasis. Although this area of research is just in its infancy, we believe that our proposed studies will shed new light on enigmatic functions of bioactive sphingolipids appropriate to their name associated with the riddle of the Sphinx. It should also provide deeper understanding of how perturbations of these fundamental biological processes contribute to human diseases.