Sepsis is a life-threatening organ dysfunction syndrome caused by a dysregulated host response to infection, Sepsis is clinically characterized by severe infection, systemic inflammatory response, and organ dysfunction, and is associated with high morbidity and mortality (1, 2). In ICU ward, clinicians usually use SOFA score to judge the prognosis of patients, the international Sepsis3.0 definition proposes that sepsis can be diagnosed when the patient reaches “infection and SOFA score ≥ 2” (3). An estimated 31.5 million cases of sepsis and 19.4 million cases of severe sepsis have been reported worldwide. Sepsis accounts for potentially 5.3 million deaths annually and the mortality rate among patients with sepsis in ICU Ward can reach 41.9% (2, 4). Incidence of sepsis will continue to rise with the increasing ages of the populations and corresponding rise in the underlying diseases (5). Neglect in the disparities in sepsis cases in developing and middle-income countries and the lack of epidemiological data on sepsis in low- and middle-income countries can lead to underestimation of incidence of sepsis and associated mortality (2, 4). The intestine is the largest immune barrier organ in the human body and is the home for a microbiome composed of more than 100 trillion species of bacteria. The host and the bacteria are mutually beneficial and symbiotic, under normal physiological conditions, the intestinal environment maintains its balance. The intestinal barrier can protect the organism from intestinal microbes and toxins (6, 7). The earlier studies have indicated that the gut is the driving force for multiple organ dysfunction in critical illness (8). Intensive research on sepsis has demonstrated intestinal barrier dysfunction to be a common complication in sepsis (9). Intestinal injury is an early marker in the development of systemic inflammatory response syndrome and multiple organ failure (MOF) (10). Sepsis causes disturbance of the gastrointestinal microenvironment, causing an imbalance between the host and the gut microbiota, which further leads to compromised intestinal barrier function and reduced intestinal immunity (11, 12). Meanwhile, studies have shown that sepsis is associated with a higher mortality rate caused by gastrointestinal injury (13).

Sphingosine-1-phosphate (S1P) is a lipid substance that is widely present in human body fluids, tissues, and cells and is responsible for conveying intercellular information as a first messenger (14). During the pathogenic process of sepsis, S1P maintains the integrity of vascular endothelial cells (ECs), promotes lymphocyte circulation, reduces systemic inflammatory responses, as well as protects the integrity of the intestinal barrier (15, 16). With the development of drugs targeting S1P and agonists and antagonists of Sphingosine-1-phosphate receptors (S1PRs), several therapeutic strategies such as FTY720, an agonist of S1PRs, which improves systemic conditions and reduces local inflammation in the intestine of mice have been proposed for treating inflammatory bowel disease (17, 18). Currently, available clinical trials have demonstrated decreased plasma concentrations of S1P in patients with sepsis compared with normal individuals (19), and have also indicated that the S1P levels are further lowered with increasing severity of sepsis (20, 21). Recent studies have confirmed that S1P alleviates LPS-induced intestinal epithelial cell injury, maintains the colonic mucosal barrier, and prevents intestinal injury during sepsis (22). S1P binding to different S1PRs activates downstream signaling pathways involved in cell proliferation, migration, apoptosis, immune regulation, anti-inflammation, and information transmission processes, showing multiple effects (14, 23). Studies have shown that the intestine is an immune organ and S1P can attenuate sepsis intestinal injury by regulating immune responses (24). Therefore, understanding the role of S1P in the septic gut is of great significance for the treatment of sepsis and sepsis-associated intestinal injury (23, 25). In this review, we have summarized the recent research progress on protective effects of the epithelial barrier and potential therapeutic targets of S1P in sepsis-associated intestinal injury.

Sphingosine 1-phosphate is synthesized either by a de novo pathway from serine and palmitoyl-CoA or produced by a the sphingomyelinase (SMase) pathway from the ubiquitous membrane lipid sphingomyelin (SM), ceramide (Cer), and sphingosine (Sph) are intermediates the sphingomyelinase in both pathways (26). Sph can be recovered by acylation, which is called “salvage pathway,” leading to regeneration of Cer (27, 28). This is followed by hydrolysis of Cer to generate Sph, and finally Sph generates S1P following the action of sphingosine kinases (SphKs) in a process that occurs in the cell membrane and cytoplasm (29) (Figure 1, By Figdraw). Ultimately, S1P is either cleaved by S1P lyase or gets dephosphorylated to Sph by S1P phosphatase (30, 31). SMase pathway begins with sphingolipids (SLs), which are ubiquitous structural components in cell membranes, SLs metabolites affect cell apoptosis, cell growth and cell migration (32). Endogenous SLs play crucial roles at multiples stages in cell biological processes and human health (33). With the development of research, it has been found that bioactive SLs, e.g., SM, Sph, Cer, S1P, and ceramide-1-phosphate may derive from dietary SLs ingested through the diet (34). Dietary SLs are ingested and absorbed in the gastrointestinal tract, affecting immune activation status, contributing to pro-inflammatory and anti-inflammatory immune responses, and can be used by cells to regulate growth, differentiation, apoptosis, and other functions (15, 35). Some studies have shown that exogenous SLs may dampen both acute and chronic inflammatory responses in cell and animal models (36, 37). According to earlier studies conducted in the last century, consumption of SLs in the United States has been estimated to be 0.3–0.4 g/day (35). SM in milk has been shown to promote gut developmental maturation (36). Dietary SM is digested by intestinal alkaline SMase (alk-SMase) and neutral ceramidase (n-CDase), and finally hydrolyzed into Cer, choline phosphate, Sph and fatty acids in the small intestine (34). Sph can be completely absorbed into intestinal mucosal cells and transformed into S1P, which is transported to lymph circulation and blood circulation together with chyle particles (38). The metabolism of SLs is affected by the type of fatty acid in diet, vitamin B6, vitamin C, vitamin D, and vitamin K (34, 39).

Sphingosine 1-phosphate is mainly derived from red blood cells, lymphatic endothelial cells and platelets (40), and S1P concentration in blood and lymph fluid is much higher than that in lymphatic tissue, forming S1P gradient (29). Sph is converted to S1P in the intestinal mucosa, where SPHK1/2 and SPL are highly expressed, and where SPL content is also higher than in any other tissue (41, 42). S1P is expressed in epithelial cells of the small intestine and colon mucosa, but the level in the small intestine is more than 2-fold higher than that in the colonic mucosa (43). Due to the abundant blood flow, lymph and lymphocytes in the intestine, which can digest food, and the presence of a large number of intestinal microorganisms, studies have found that dietary SM is mainly destroyed into sphingosine and sphinganine in the jejunum through a large amount of alk-SMase and nCDase released by the intestinal epithelium and liver, which are rapidly absorbed and metabolized by intestinal mucosal cells to generate S1P (44). SL metabolites (including S1P) can also be produced by gut microbes and directly affect host metabolic pathways, for example, Bacteroides fragilis (15, 45).

Sphingosine 1-phosphate is a bioactive sphingolipid that is present in high-density lipoprotein (HDL), and the main binding carrier is apolipoprotein M (apoM). In plasma, 60% of S1P is normally bound to apoM and 40% binds to albumin (46). S1P needs to bind to these carriers to be transported in the biological fluids (47). Moreover, S1P, being a small polar phospholipid, is transported through the lipid bilayer in a paracrine or autocrine manner, with the aid of the transporter ATP-binding cassette (ABC) transporters and spinster homolog 2 (SPNS2) (48).

Sphingolipids are metabolized in the intestine and are primarily found in the mucosal cells of the small intestine and colon, where their metabolites are absorbed and enter the blood circulation or lymph (39). S1P, which is widely found in the blood and lymph, is more abundant in the gut than in other tissues, and influences the immune status of the intestines (29, 43). The maximum concentration of S1P in circulation cells is found in erythrocytes, EC, immune cells, and platelets, as the activities of S1P lyase and phosphatase are weak within erythrocytes, which ensure that the high concentrations of S1P are maintained (49). Lymphatic ECs are the main source of S1P in the lymph (29). Multiple stimuli during sepsis regulate key enzymes, thereby affecting sphingolipid biosynthesis and metabolism, resulting in restricted S1P production (29). The finding in an earlier study, according to which the S1P levels in the blood of patients with sepsis were lower than normal, could be attributed to the fact that apoM levels are reduced in patients with sepsis and systemic inflammatory response syndrome; because of which S1P plasma levels are also lower (50). In an experiment on sepsis in humans and baboons, it was observed that S1P levels were lower in most septic groups, when compared to normal controls, less by almost 46% in severe septic shock, and a homogeneous distribution of S1P was observed between apoM and albumin. A significant correlation was observed between S1P decline and apoM during sepsis (20). Because hepatocytes express and secrete the majority of apoM and albumin, serum S1P levels were found to decrease sharply with advanced liver disease and predict early death in patients with chronic liver disease, and low S1P levels may significantly affect the progression of multiple organ dysfunction syndrome (MODS) (51). Makoto Kurano’s experiment demonstrated that while knockout or knockdown of apoM aggravated lethality and organ damage in lipopolysaccharide (LPS) mice, apoM/S1P improved survival and protected organs by preventing apoptosis (52). While erythrocytes are the major source of S1P in plasma, during sepsis, they also contribute to iatrogenic blood loss, reduced plasma iron, suppressed erythropoietin production and shortened RBC lifespan as well as malnutrition (53), which in turn could account for the reduced S1P levels. Another important cause for reduced S1P levels in sepsis patients could be the fact that sepsis causes acute damage and dysfunction of the gastrointestinal tract, which results in insufficiency of nutritional sources for the patients (54), thereby affecting their nutritional status. However, the baboon model demonstrated no correlation between the number of erythrocytes and S1P. In contrast, platelet levels of S1P in humans closely correlate with those in the baboon sepsis (20).

Human megakaryocytes (MKs) release trillions of platelets into the circulation every day to maintain platelet levels. Studies have revealed that S1PR1 signaling continuously inhibits MKs growth, and when S1P metabolism is disrupted in the hematopoietic environment, S1PR2 signaling can further inhibit thrombosis. S1P generation is related to SphK1 activation (55).

Studies have shown that SphK1 is 15 times higher than SphK2 in normal human body, while the expression of SphK1 in platelets of sepsis patients is significantly reduced, while SphK2 has no significant change (56). The decreased activity of Sphk1 in cells will promote the production of platelets, and the metabolic pathway of S1P production is related to the disease of thrombocytopenia (55). In addition, recent studies have shown that platelets store S1P in the cytoplasm in the resting state, and after platelets are activated, this S1P pool is transported to the plasma membrane to regulate local cellular responses, in which the loss of the major facilitator superfamily transporter 2b (Mfsd2b) of S1P reduces platelet thrombosis (57, 58). In platelets, Sphk1 is responsible for S1P production, and Mfsd2b is responsible for S1P export. It has been shown that platelets release S1P to protect the heart during acute myocardial infarction (AMI), and antiplatelet strategy to preserve platelet S1P release during AMI is the most ideal strategy (59). After acute infection, the activation of coagulation and inflammation is the key cascade, leading to thromboinflammation and microthrombosis. The intestinal tract of septic mice shows strong inflammatory and thrombotic reactions (60), and severe imbalance of coagulation and anticoagulation can lead to disseminated intravascular coagulation (DIC) and death. DIC, as an extreme state of systemic coagulation activation, leads to excessive consumption of platelets due to continuous increase in coagulation activity after its appearance, and eventually forms thrombocytopenia (61). Thrombin has been considered as a potential therapeutic target for sepsis, and studying the thrombotic effects of Mfsd2b and S1P on platelets may be helpful for the treatment of the pathological process of sepsis in the future (62, 63).

It has been reported that SphK1 is 15-fold higher than SphK2 in normal humans, whereas in platelets from sepsis patients, SphK1 expression is significantly reduced with no change in SphK2 Recent studies indicate that platelets produce a large amount of S1P through de novo synthesis, which is released after platelet activation, thereby modulating local cellular responses (57). Thus, platelets provide S1P for a short time during sepsis, which may be associated with the activation of SphK1.

Sphingosine 1-phosphate, a metabolite of cell membrane sphingolipids, was reported to be a second messenger that mediates increases in intracellular calcium levels, which transmit signals by binding to S1PRs on the cell membrane surface. However, S1P was also unexpectedly found to function as an intercellular first messenger (64, 65). The five receptors for S1P are G protein-coupled receptor subtypes, and all immune cells express the unique characteristics of S1PRs, which are involved in adaptive immune cell trafficking, angiodevelopment, and homeostasis (14). For T-cells, S1PR1 signals tonically reduce T-cell chemotactic sensitivity to chemokines and thereby limit homing of blood and spleen T-cells to secondary lymphoid tissues (66).

Sphingosine-1-phosphate receptors are differentially expressed in various tissues. S1PR1 is located in most organs, and is expressed at high levels during embryonic development, vascular production and maturation, bone remodeling, and in the immune system (67). S1PR2 plays important roles in inhibiting apoptosis and proliferation, actin remodeling, and development of the cardiovascular, visual and auditory systems (23). Studies have shown that S1PR2 is highly expressed in intestinal epithelial cells (IECs) and promotes their proliferation (68). S1PR3 is found in most organ tissues, and current research focuses on cardiovascular issues, sepsis, cardiac conduction, stroke, and cancer metabolism (69). S1PR4 is mainly expressed in immune organs, and is involved in megakaryocyte differentiation and platelet development (70), and S1PR5 is expressed at high levels in spleen, skin, lung, and brain, plays a role in myelination, and is involved in nervous system expression (71, 72). The expression of S1PRs in intestinal endothelial cells is low, but a variety of S1PR-targeted drugs have high affinity for S1PR1 and S1PR5. Ozanimod, for example, has been approved for the first time in the United States, European Union and other countries for the treatment of moderate to severe active ulcerative colitis in adults, the main mechanism is to bind to S1PR1 and S1PR5 receptors. Internalization of S1PR1 reduces the ability of lymphocytes to secrete from lymphoid tissues (73). In addition, gut microbes affect gut and brain pathophysiology through the microbiome-gut-brain axis (MGBA). The gut microbiome of patients with sepsis may contribute to the pathogenesis of sepsis-associated encephalopathy (SAE). The study of septic intestinal injury will also help researchers decipher SAE (74).

The S1P-S1PR signaling system contributes to the regulation of complex inflammatory processes by influencing lymphocyte trafficking and maintenance of vascular integrity. thereby S1PR1-3 is mainly expressed by ECs, and S1PR5 is expressed only by blood-brain-barrier cells (16). Under inflammatory conditions, S1PR1-3 are widely distributed, with the highest expression levels in the cardiovascular and immune systems while S1PR4 and S1PR5 are relatively less expressed (75). The downstream signaling of these receptors is very complex because they are expressed differently in ECs and are coupled to multiple G proteins. For example, S1P in the blood circulation binds to G protein-coupled receptors on the cell membrane of intestinal ECs (Figure 2, By Figdraw). After coupling with the G αi/o, S1P-S1PR1 activates the downstream Phosphoinositide 3-kinase (PI3K), phospholipase C (PLC), and Rac families, leading to Rac-dependent cytoskeletal rearrangements and cell matrix contact. It also affects the adherens junction assembly, and barrier integrity, thereby enhancing the EC barrier (71, 76). However, S1PR2 and S1PR3 bind to G αi/o, G αq, and G α_12/13, which combined with G αq activate PLC and increase the intracellular calcium concentration, leading to vasoconstriction, decreased vascular permeability, and weakened barrier function. Both S1PR2 and S1PR3, upon binding with G α_12/13, activate the Rho family to promote the transition of ECs to a contractile phenotype, and attenuate inter-endothelial junctions. Both pathways disrupt the capillary endothelial barrier by increasing vascular permeability. Interestingly, the two pathways mentioned above have the exactly opposite effect of combining G αi/o production (71, 77). S1PR4 and S1PR5 bind to the G α_i/o, G α_12/13 protein pathway, causing weakening of the EC barrier. The differential but overlapping expression patterns of the five receptors form the molecular basis for the different S1P functions.

A growing number of studies have demonstrated that S1P plays an important role in the vasculature, inflammatory response, immune response, and maintenance of the intestinal mucosal barrier, and is considered a potential therapeutic biomarker for sepsis. This might provide a new direction for the development of targeted therapeutics for sepsis by targeting the S1P-S1PR signaling mechanism (Table 1) (19, 126). Four S1PR modulators (fingolimod, siponimod, ozanimod, and ponesimod) have regulatory approval for multiple sclerosis. Targeting S1PRs for the treatment of inflammatory diseases has shown successful results in clinical trials, and they are mainly indicated for treating inflammatory bowel diseases (IBD), including ulcerative colitis and Crohn’s disease (151). The immunomodulator FTY720 is a structural analogue of S1P and acts in its phosphorylated isoform as an unselective agonist on S1PR1 and S1PR3-5 and a selective functional antagonist on S1PR1 (152). Fingolimod (FTY720), the most investigated modulator, is efficacious on all subtypes except S1PR2. It inhibits S1PR1 expression after activation of S1PR1 on lymphocytes and limits the egress of lymphocytes from lymphoid tissues, thus reducing lymphocytic infiltration and reducing the extent of damage, and ameliorating local inflammation in the intestine in murine colitis (17, 18). FTY720 improved sepsis incidence and reduced sepsis-induced hypothermia and weight loss. Lymphopenia leads to the accumulation of lymphocytes in peripheral lymph nodes and reduces the bacterial load in the liver, and proinflammatory factors such as plasma IL-6, TNF-α, MCP-1, and IL-10 are significantly reduced (153). In acute lung injury, sepsis-associated encephalopathy and cardiac function injury, FTY720 can be seen to show a protective phenotype in experimental sepsis by regulating vascular and immune functions (154–156). In the latest DSS-induced colitis model study, it was confirmed that FTY720 improved abnormal immune response by capturing T cells and inhibiting the polarization of M1 macrophages in mice with colitis, and this effect has not been clarified in sepsis (157). Previous studies reported that S1P regulates peritoneal B cell trafficking and subsequent intestinal IgA production, and FTY720 significantly reduced the production of natural gut secretory IgA derived from peritoneal B cells, demonstrating the critical role of S1P in intestinal cellular immunity (158). This study found that there is the production of Th17 proinflammatory cells during intestinal inflammation, and FTY720 directly inhibited the differentiation of Th17 cells in vitro, while increasing Treg differentiation from naive CD4 + T cells and inhibiting IL-23-mediated activation of STAT4, NF-kB and AKT. In addition, it also inhibited Dectin-1 expression in mature and immature monocyte-derived dendritic cells, which in turn inhibited curdlan-mediated production of IL-23p19, p40, IL-6, and IL-1β cytokines (159). Lymphopenia (T and B cells) induced by FTY720 did not negatively affect mortality from sepsis during challenge with severe abdominal sepsis (160).

The discovery of the S1PR3 receptor mediating bradycardia in mice prompted the search for modulators devoid of S1PR3 signaling (161). The published study demonstrating that the Sphingosine 1-phosphate receptor modulator, siponimod, does not just ameliorate the inflammatory aspect but also the degenerative aspect of secondary progressive MS (162).

Ozanimod, which attenuates immune responses by reducing circulating lymphocytes, has been shown to reduce inflammation and disease severity in animal models of colitis (163). Phase III trials are currently being conducted to test its clinical efficacy in ulcerative colitis, clinical trials on Ozanimod intervention in COVID-19 are also ongoing (25, 164, 165), Furthermore, under conditions of non-interference with protective immunity, targeting specific T lymphocyte subsets of autoimmune disease by ozanimod improves the disease situation (166). Recently, ponesimod another receptor modulator has been highlighted for its safety and efficacy by the continuous research on autoimmune diseases, which specifically activate signal transduction through the S1PR1 pathway (167). Ponesimod can prevent inflammation due to autoimmune reactions and can also cross the blood-brain barrier hindering synaptic neurodegeneration, thus exerting neuroprotective behavior (168, 169).

Since S1P exhibits multiple protective effects in septic intestinal injury, the fatality rate associated with septic intestinal injury can be reduced by elevating S1P levels or using S1PRs modulators (19). With regards to the experimental observation of lowered S1P levels in the plasma of patients with sepsis, some studies have demonstrated that intravenous injection of exogenous S1P into mice caused a transient increase in plasma S1P levels. But this exogenous S1P was cleared out within 15–30 min, because of the short biological half-life of S1P. It was also observed that the S1P degradation was related to the cellular location since intracellular S1P was degraded by SPL, while the extracellular degradation was not related to SPL (170). Treatment of sepsis by direct injection of S1P is not yet feasible, and increasing S1P levels in plasma may either enhance S1P generation by SphKs, or reduce S1P degradation (19). It is important to explore the mechanism through which S1P functions through the immune response for the treatment of sepsis (24). Based on current clinical data on S1PRs modulators, they are thought to reduce inflammation in immune-mediated diseases mainly by blocking lymphocyte egress from lymph nodes to the bloodstream (151, 171). To date, S1PR modulators have made great progress in the treatment of autoimmune intestinal inflammatory diseases, and some molecules have already been approved for use with proven safety and efficacy (169), however, the efficacy of these agents in treating sepsis-associated intestinal injury requires further investigation (23). In addition, the present study found inter-individual functional differences between SphKs and S1PRs, for example, SphK2 inhibits the production of anti-inflammatory factor IFN-β, in contrast to action exerted by SphK1 (102) and S1PR1 exerts anti-inflammatory effects, whereas S1PR2 accelerates the induction of TNF-α activation, which promotes inflammatory responses (172, 173). Therefore, there is a continuous effort to develop new drugs that target the function of different kinases with receptor subtypes, with the aim of providing new therapeutic targets for sepsis-associated intestinal injury (23).

Sphingosine 1-phosphate as one of the EC barrier reinforcing molecules plays an important role in maintaining barrier integrity. In the pathogenic process of sepsis, S1P reduces the case fatality rate associated with sepsis by suppressing the storm of the inflammatory factor. The gut, as the prime factor in MODS, is often attacked by sepsis bacteria with toxins, causing sepsis-associated intestinal injury and dysfunction. S1P binds to different S1PRs to play different roles, for example, the S1P-S1PR1 pathway regulates multiple immune responses and decreases inflammatory responses and is an effective target for the treatment of sepsis. In intestinal injury, S1PR1 alleviates the inflammatory storm and is involved in diverse immune cell migration. The complex regulatory network of S1P signaling, metabolism, and trafficking have made it difficult to translate the present findings translated into clinical applications. Understanding the mechanisms by which intestinal inflammation, immune responses, and S1P interact could help in the development of novel targets with an aim to reduce the morbidity and mortality associated with intestinal injury in sepsis. In the future we need to explore the signaling pathways of S1P on individual S1PRs in the gut under sepsis conditions and observe the mechanism by which the gut microbiota regulates S1P levels. Whether S1PR modulators can be effective targets for intestinal injury in sepsis requires further study. Because of the short biological half-life of S1P, the current available experimental evidence does not support the direct injection of S1P. The currently developed treatment modalities involving receptor modulators show good stability versus effectiveness. Developing highly selective modulators of S1PRs are the future research directions. Finally, the mechanisms of S1P signaling could provide a high-value regimen superior to traditional therapy for combating sepsis-associated intestinal injury, providing a new avenue for exploring the pathogenesis of the disease and implementing individualized therapeutic strategies.

GS: Writing—original draft. BW: Writing—original draft. HZ: Writing—review and editing. JY: Writing—review and editing. XL: Writing—review and editing.