Mfsd2b and Spns2 are essential for maintenance of blood vessels during development and in anaphylactic shock

Discovery of Potent, Orally Bioavailable Sphingosine-1-Phosphate Transporter (Spns2) Inhibitors

Targeting the S1P pathway has resulted in the development of S1P1 receptor modulators for the treatment of multiple sclerosis and ulcerative colitis. We hypothesize that targeting an upstream node of the S1P pathway may provide an improved adverse event profile. In this report, we performed a structure-activity relationship study focusing on the benzoxazole scaffold in SLB1122168, which lead to the discovery of 11i (SLF80821178) as a potent inhibitor of S1P release from HeLa cells (IC50: 51 ± 3 nM). Administration of SLF80821178 to mice induced ∼50% reduction in circulating lymphocyte counts, recapitulating the lymphopenia characteristic of Spns2 null animals. Molecular modeling studies suggest that SLF80821178 binds Spns2 in its occluded inward-facing state and forms hydrogen bonds with Asn112 and Ser211 and π stacking with Phe234. Taken together, SLF80821178 can serve as a scaffold for future inhibitor development and represents a chemical tool to study the therapeutic implication of inhibiting Spns2.

Regulation of cellular and systemic sphingolipid homeostasis

One hundred and fifty years ago, Johann Thudichum described sphingolipids as unusual "Sphinx-like" lipids from the brain. Today, we know that thousands of sphingolipid molecules mediate many essential functions in embryonic development and normal physiology. In addition, sphingolipid metabolism and signalling pathways are dysregulated in a wide range of pathologies, and therapeutic agents that target sphingolipids are now used to treat several human diseases. However, our understanding of sphingolipid regulation at cellular and organismal levels and their functions in developmental, physiological and pathological settings is rudimentary. In this Review, we discuss recent advances in sphingolipid pathways in different organelles, how secreted sphingolipid mediators modulate physiology and disease, progress in sphingolipid-targeted therapeutic and diagnostic research, and the trans-cellular sphingolipid metabolic networks between microbiota and mammals. Advances in sphingolipid biology have led to a deeper understanding of mammalian physiology and may lead to progress in the management of many diseases.

Assessing Spns2-dependent S1P Transport as a Prospective Therapeutic Target

S1P (sphingosine 1-phosphate) receptor modulator (SRM) drugs interfere with lymphocyte trafficking by downregulating lymphocyte S1P receptors. While the immunosuppressive activity of SRM drugs has proved useful in treating autoimmune diseases such as multiple sclerosis, that drug class is beset by on-target liabilities such as initial dose bradycardia. The S1P that binds to cell surface lymphocyte S1P receptors is provided by S1P transporters. Mice born deficient in one of these, spinster homolog 2 (Spns2), are lymphocytopenic and have low lymph S1P concentrations. Such observations suggest that inhibition of Spns2-mediated S1P transport might provide another therapeutically beneficial method to modulate immune cell positioning. We report here results using a novel S1P transport blocker (STB), SLF80821178, to investigate the consequences of S1P transport inhibition in rodents. We found that SLF80821178 is efficacious in a multiple sclerosis model but - unlike the SRM fingolimod - neither decreases heart rate nor compromises lung endothelial barrier function. Notably, although Spns2 null mice have a sensorineural hearing defect, mice treated chronically with SLF80821178 have normal hearing acuity. STBs such as SLF80821178 evoke a dose-dependent decrease in peripheral blood lymphocyte counts, which affords a reliable pharmacodynamic marker of target engagement. However, the maximal reduction in circulating lymphocyte counts in response to SLF80821178 is substantially less than the response to SRMs such as fingolimod (50% vs. 90%) due to a lesser effect on T lymphocyte sub-populations by SLF80821178. Finally, in contrast to results obtained with Spns2 deficient mice, lymph S1P concentrations were not significantly changed in response to administration of STBs at doses that evoke maximal lymphopenia, which indicates that current understanding of the mechanism of action of S1P transport inhibitors is incomplete.

Role of sphingosine 1-phosphate (S1P) in sepsis-associated intestinal injury

Sphingosine-1-phosphate (S1P) is a widespread lipid signaling molecule that binds to five sphingosine-1-phosphate receptors (S1PRs) to regulate downstream signaling pathways. Sepsis can cause intestinal injury and intestinal injury can aggravate sepsis. Thus, intestinal injury and sepsis are mutually interdependent. S1P is more abundant in intestinal tissues as compared to other tissues, exerts anti-inflammatory effects, promotes immune cell trafficking, and protects the intestinal barrier. Despite the clinical importance of S1P in inflammation, with a very well-defined mechanism in inflammatory bowel disease, their role in sepsis-induced intestinal injury has been relatively unexplored. In addition to regulating lymphocyte exit, the S1P-S1PR pathway has been implicated in the gut microbiota, intestinal epithelial cells (IECs), and immune cells in the lamina propria. This review mainly elaborates on the physiological role of S1P in sepsis, focusing on intestinal injury. We introduce the generation and metabolism of S1P, emphasize the maintenance of intestinal barrier homeostasis in sepsis, and the protective effect of S1P in the intestine. We also review the link between sepsis-induced intestinal injury and S1P-S1PRs signaling, as well as the underlying mechanisms of action. Finally, we discuss how S1PRs affect intestinal function and become targets for future drug development to improve the translational capacity of preclinical studies to the clinic.

An erythrocyte-centric view on the MFSD2B sphingosine-1-phosphate transporter

MFSD2B has been identified as the exclusive sphingosine-1-phosphate (S1P) transporter in red blood cells (RBC) and platelets. MFSD2B-mediated S1P export from platelets is required for aggregation and thrombus formation, whereas RBC MFSD2B maintains plasma S1P levels in concert with SPNS2, the vascular and lymphatic endothelial cell S1P exporter, to control endothelial permeability and ensure normal vascular development. However, the physiological function of MFSD2B in RBC remains rather elusive despite mounting evidence that the intracellular S1P pool plays important roles in RBC glycolysis, adaptation to hypoxia and the regulation of cell shape, hydration, and cytoskeletal organisation. The large accumulation of S1P and sphingosine in MFSD2B-deficient RBC coincides with stomatocytosis and membrane abnormalities, the reasons for which have remained obscure. MFS family members transport substrates in a cation-dependent manner along electrochemical gradients, and disturbances in cation permeability are known to alter cell hydration and shape in RBC. Furthermore, the mfsd2 gene is a transcriptional target of GATA together with mylk3, the gene encoding myosin light chain kinase (MYLK). S1P is known to activate MYLK and thereby impact on myosin phosphorylation and cytoskeletal architecture. This suggests that metabolic, transcriptional and functional interactions may exist between MFSD2B-mediated S1P transport and RBC deformability. Here, we review the evidence for such interactions and the implications for RBC homeostasis.

2-Aminobenzoxazole Derivatives as Potent Inhibitors of the Sphingosine-1-Phosphate Transporter Spinster Homolog 2 (Spns2)

The S1P1 receptor is the target of four marketed drugs for the treatment of multiple sclerosis and ulcerative colitis. Targeting an S1P exporter, specifically Spns2, that is "upstream" of S1P receptor engagement is an alternate strategy that might recapitulate the efficacy of S1P receptor modulators without cardiac toxicity. We recently reported the first Spns2 inhibitor SLF1081851 (16d) that has modest potency with in vivo activity. To develop more potent compounds, we initiated a structure-activity relationship study that identified 2-aminobenzoxazole as a viable scaffold. Our studies revealed SLB1122168 (33p), which is a potent inhibitor (IC50 = 94 ± 6 nM) of Spns2-mediated S1P release. Administration of 33p to mice and rats resulted in a dose-dependent decrease in circulating lymphocytes, a pharmacodynamic indication of Spns2 inhibition. 33p provides a valuable tool compound to explore both the therapeutic potential of targeting Spns2 and the physiologic consequences of selective S1P export inhibition.

Postnatal deletion of Spns2 prevents neuroinflammation without compromising blood vascular functions

Protein Spinster homolog 2 (Spns2) is a sphingosine-1-phosphate (S1P) transporter that releases S1P to regulate lymphocyte egress and trafficking. Global deletion of Spns2 (Spns2^-/-) has been shown to reduce disease severity in several autoimmune disease models. To examine whether Spns2 could be exploited as a drug target, we generated and characterized the mice with postnatal knockout of Spns2 (Spns2-Mx1Cre). Our results showed that Spns2-Mx1Cre mice had significantly low number of lymphocytes in blood and lymphoid organs similar to Spns2^-/- mice. Lymph but not plasma S1P levels were significantly reduced in both groups of knockout mice. Our lipidomic results also showed that Spns2 releases different S1P species into lymph. Interestingly, lymphatic vessels in the lymph nodes (LNs) of Spns2^-/- and Spns2-Mx1Cre mice exhibited morphological defects. The structures of high endothelial venules (HEV) in the LNs of Spns2-Mx1Cre mice were disorganized. These results indicate that lack of Spns2 affects both S1P secretion and LN vasculatures. Nevertheless, blood vasculature of these Spns2 deficient mice was not different to controls under homeostasis and vascular insults. Importantly, Spns2-Mx1Cre mice were resistant to multiple sclerosis in experimental autoimmune encephalomyelitis (EAE) models with significant reduction of pathogenic Th17 cells in the central nervous system (CNS). This study suggests that pharmacological inhibition of Spns2 may be exploited for therapeutic applications in treatment of neuroinflammation.