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

Assessment of Spinster homologue 2 (Spns2)-dependent transport of sphingosine-1-phosphate as a therapeutic target

BACKGROUND AND PURPOSE

Sphingosine-1-phosphate (S1P) receptor modulator (SRM) drugs suppress immune system function by disrupting lymphocyte trafficking, but SRMs are broadly immunosuppressive with on-target liabilities. Another strategy to modulate the immune system is to block S1P transport. This study tests the hypothesis that blockers of S1P transport (STBs) mediated by Spinster homologue 2 (Spns2) approximate the efficacy of SRMs without their adverse events.

EXPERIMENTAL APPROACH

We have discovered and optimized STBs to enable investigations of S1P biology and to determine whether S1P transport is a valid drug target. The STB SLF80821178 was administered to rodents to assess its efficacy in a multiple sclerosis model and to test for toxicities associated with SRMs or Spns2-deficient mice. Further, potential biomarkers of STBs, absolute lymphocyte counts (ALCs) in blood and S1P concentrations in plasma and lymph, were measured.

KEY RESULTS

SLF80821178 resembles SRMs in that it is efficacious in a standard multiple sclerosis model but does not evoke bradycardia or lung leakage, common to the SRM drug class. Also, chronic SLF80821178 administration does not affect auditory responses in adult mice despite the neurosensorial hearing defect observed in Spns2-null mice. While both SRM and STB administration decrease ALCs, the maximal effect is less with an STB (45% vs. 90%). STBs have minimal effects on S1P concentration in plasma or thoracic duct lymph.

CONCLUSION AND IMPLICATIONS

We found nothing to invalidate Spns2-dependent S1P transport as a drug target. Indeed, STBs could be superior to SRMs as a therapy to modulate immune system function.

Zonation and ligand and dose dependence of sphingosine 1-phosphate receptor-1 signalling in blood and lymphatic vasculature

AIMS

Circulating levels of sphingosine 1-phosphate (S1P), an HDL-associated ligand for the endothelial cell (EC) protective S1P receptor-1 (S1PR1), are reduced in disease states associated with endothelial dysfunction. Yet, as S1PR1 has high affinity for S1P and can be activated by ligand-independent mechanisms and EC autonomous S1P production, it is unclear if relative reductions in circulating S1P can cause endothelial dysfunction. It is also unclear how EC S1PR1 insufficiency, whether induced by deficiency in circulating ligand or by S1PR1-directed immunosuppressive therapy, affects different vascular subsets.

METHODS AND RESULTS

We here fine map the zonation of S1PR1 signalling in the murine blood and lymphatic vasculature, superimpose cell-type-specific and relative deficiencies in S1P production to define ligand source and dose dependence, and correlate receptor engagement to essential functions. In naïve blood vessels, despite broad expression, EC S1PR1 engagement was restricted to resistance-size arteries, lung capillaries, and a subset of high-endothelial venules (HEVs). Similar zonation was observed for albumin extravasation in EC S1PR1-deficient mice, and brain extravasation was reproduced with arterial EC-selective S1pr1 deletion. In lymphatic ECs, S1PR1 engagement was high in collecting vessels and lymph nodes and low in blind-ended capillaries that drain tissue fluids. While EC S1P production sustained S1PR1 signalling in lymphatics and HEV, haematopoietic cells provided ∼90% of plasma S1P and sustained signalling in resistance arteries and lung capillaries. S1PR1 signalling and endothelial function were both surprisingly sensitive to reductions in plasma S1P with apparent saturation around 50% of normal levels. S1PR1 engagement did not depend on sex or age but modestly increased in arteries in hypertension and diabetes. Sphingosine kinase (Sphk)-2 deficiency also increased S1PR1 engagement selectively in arteries, which could be attributed to Sphk1-dependent S1P release from perivascular macrophages.

CONCLUSION

This study highlights vessel subtype-specific S1PR1 functions and mechanisms of engagement and supports the relevance of S1P as circulating biomarker for endothelial function.

Spinster homolog 2 (SPNS2) deficiency drives endothelial cell senescence and vascular aging via promoting pyruvate metabolism mediated mitochondrial dysfunction

Endothelial cell (EC) senescence and vascular aging are important hallmarks of chronic metabolic diseases. An improved understanding of the precise regulation of EC senescence may provide novel therapeutic strategies for EC and vascular aging-related diseases. This study examined the potential functions of Spinster homolog 2 (SPNS2) in EC senescence and vascular aging. We discovered that the expression of SPNS2 was significantly lower in older adults, aged mice, hydrogen peroxide-induced EC senescence models and EC replicative senescence model, and was correlated with the expression of aging-related factors. in vivo experiments showed that the EC-specific knockout of SPNS2 markedly aggravated vascular aging by substantially, impairing vascular structure and function, as evidenced by the abnormal expression of aging factors, increased inflammation, reduced blood flow, pathological vessel dilation, and elevated collagen levels in a naturally aging mouse model. Moreover, RNA sequencing and molecular biology analyses revealed that the loss of SPNS2 in ECs increased cellular senescence biomarkers, aggravated the senescence-associated secretory phenotype (SASP), and inhibited cell proliferation. Mechanistically, silencing SPNS2 disrupts pyruvate metabolism homeostasis via pyruvate kinase M (PKM), resulting in mitochondrial dysfunction and EC senescence. Overall, SPNS2 expression and its functions in the mitochondria are crucial regulators of EC senescence and vascular aging.

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.

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.

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.