Function and Modulation of Sphingosine-1-Phosphate Receptors in the Central Nervous System

Abstract

Sphingolipids, first discovered in 1874 by Johann Thudicum, are among the eight recognized classes of lipids and are present in essentially all plants, animals, and fungi, as well as some viruses and prokaryotes. In mammals, sphingolipids are enriched in the central nervous system (CNS), where they play vital roles in tissue development; membrane structure; cell adhesion and recognition; and, importantly, signaling. A subset of sphingolipids including ceramide, glucosylceramide, and sphingosine has been shown to have bioactive properties, but two sphingolipids in particular (ceramide-1-phosphate and sphingosine-1-phosphate) have been shown to exert their effects at least in part due to the activation of cell surface-expressed G protein-coupled receptors. In the CNS, sphingosine-1-phosphate signaling has specifically emerged as a productive therapeutic target for the treatment of neurodegenerative disease, with the first small molecule targeting sphingosine-1-phosphate receptors approved roughly 15 years ago for the treatment of multiple sclerosis. As more specific activators and inhibitors of these receptors have been developed and entered the clinical trial pipeline, now is an appropriate time to examine the current state of our knowledge of the role that these receptors play in the CNS and highlight the current landscape of available modulators targeting these pathways.

1. Introduction

Sphingolipids (SLs) are a family of lipids that contain a fatty amino alcohol base called sphingosine. Sphingolipid metabolism is a tightly controlled process with only one entry point (de novo biosynthesis) and one exit point (the degradation of S1P by SGPL1 (sphinganine-1-phosphate lyase). Despite this limitation, there are estimated to be over 4000 chemically distinct species of naturally occurring sphingolipids generated through interconversion within the constrained system [1]. Ceramide (Cer) is recognized as the central SL species, serving as the building block of all complex SLs, and the metabolism of SLs is broken down into two main pathways—the de novo pathway and the salvage or recycling pathway (Figure 1). The salvage pathway functions to recycle already generated SLs back into Cer, preserving the sphingoid base for subsequent conversion back into complex SLs [2]. Within the salvage pathway is the sphingomyelin hydrolysis pathway, given its name in large part due to its long-known importance in the process of cell signaling. First described over 30 years ago, this process is initiated by sphingomyelinase-mediated hydrolysis of SM at the plasma membrane, generating Cer [3]. It is important to note that the SM utilized within the sphingomyelin hydrolysis pathway must, at one point in time, have been generated from Cer, and the resulting Cer after SM degradation is available for recycling back into complex sphingolipids or sphingosine (Sph), demonstrating why the sphingomyelin hydrolysis pathway should be viewed as a component of the larger salvage pathway (Figure 1).

The de novo biosynthesis of SLs is mediated by the serine palmitoyl transferase enzyme, which facilitates the condensation of serine and palmitoyl-CoA, generating labile intermediate 3-keto-sphinganine (3KS). 3KS is rapidly converted to dihydrosphingosine (dhSph) via 3-ketosphinganine reductase. Typically, dhSph is amino-acylated by one of six ceramide synthases (CerS) with a fatty acid of varying chain length to form dihydroceramide (dhCer). The final step of de novo synthesis occurs when one of two dihydroceramide desaturases adds a double bond between C4 and C5 in the sphingoid base to generate Cer. To generate Sph, the molecule for which the family is named, the amino-acylated fatty acid is cleaved by ceramidase. Sph can then either be converted back to Cer by CerS or be phosphorylated by one of two sphingosine kinases (SphKs) to form bioactive lysophospholipid S1P [4]. The fate of S1P is often tied to the SphK isoform that phosphorylates it [5].

These two enzymes share the same biochemical function yet exhibit differential tissue expression patterns and subcellular localization, driving their biological effects [6]. Generally, SphK1 can be found predominantly in the cytoplasm or in association with the cytoskeleton, endoplasmic reticulum (ER), Golgi apparatus, or plasma membrane, while SphK2 is mainly expressed in the nucleus and ER [7,8]. The main isoform responsible for the effect of inside-out signaling by S1P is SphK1, which is rapidly recruited to the plasma membrane during receptor-mediated endocytosis [9]. Inhibiting the activity of SphK1 results in the accumulation of dilated late endosomes and eventual cellular death in a TP53-dependent manner, highlighting SphK1’s regulatory role in membrane processing after signaling events and other endocytic processes [9,10]. Many of the biological effects of S1P can be attributed to autocrine and paracrine signaling through the sphingosine-1-phosphate receptors (S1PRs), which belong to the G Protein-Coupled Receptor (GPCR) family and are also referred to as Endothelial Differentiation Gene (EDG) receptors. There are five isotypes of S1PRs: S1PR1 (EDG-1), S1PR2 (EDG-5), S1PR3 (EDG-3), S1PR4 (EDG-6), and S1PR5 (EDG-8) [11]. While S1P signaling plays a vital role in the cardiovascular, immune, gastrointestinal, and integumentary systems, this review focuses on mammalian S1PR biology in the central nervous system, with emphasis on receptor subtype-specific signaling, structural determinants of selectivity, and therapeutic modulation relevant to CNS disease.

2. G Protein-Coupled Receptors

The G protein-coupled receptor (GPCR) family is the largest receptor family in the human genome, with roughly 800 identified members [12]. GPCRs respond to a variety of stimuli, including neurotransmitters, hormones, bioactive lipids, and even photons [13,14,15,16]. As a result of this diversity, GPCRs are heavily involved in the regulation of physiology and the development of pathophysiology. All GPCRs contain seven transmembrane domains; an extracellular N-terminal ligand recognition domain; and a triple-looped C-terminal tail, which regulates interactions with transducer proteins and effectors [12,17]. The most common of these transducers are the heterotrimeric G proteins, which are composed of a complex of Gα-, Gβ-, and Gγ-protein subunits. Binding of a ligand to these receptors activates the receptors’ intrinsic guanine nucleotide exchange factor (GEF) activity, which exchanges guanine diphosphate (GDP) bound to the Gα subunit and replaces it with guanine triphosphate (GTP), causing the disassociation of the Gα subunit from the remaining Gβ, and Gγ complex (hereafter referred to as Gβγ). Once dissociated, Gα and Gβγ transduce their signals through separate effector pathways until the GTP bound to the Gα subunit is hydrolyzed to GDP via intrinsic activity on the subunit itself or via the regulator of G-protein signaling (RGS) proteins [18]. While diversity in the Gβγ complex and the γ subunit specifically has recently become a focus of interest, this work will focus on differences in the Gα subunit family associated with S1PRs [19].

3. Gα Protein Subunit Families

There are four main families of Gα subunits (Gαs, Gαi, Gαq/11, and Gα12/13), each of which transduces its signals through a variety of effectors. Two of the families (Gαs and Gαi) regulate the activity of adenylyl cyclase (AC), which converts adenosine triphosphate (ATP) to cyclic adenosine monophosphate. Gαs stimulates AC activity, increasing the intracellular concentration of cAMP, which, in turn, activates cAMP-regulated proteins, including protein kinase A (PKA) [20]. Alternatively, Gαi inhibits AC activity, reducing cAMP activity and inhibiting PKA [21]. The Gαq/11 family activates β-isoforms of phospholipase C (PLCβ), generating inositol triphosphate (IP₃) via the cleavage of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂]. Increased intracellular levels of IP₃ induce calcium signaling through receptors on the endoplasmic reticulum (ER) [22]. The Gα12/13 subunit activates Ras homolog family member A (RhoA), a small GTPase that works through a variety of target proteins in a cell type-specific manner [23]. This promiscuous nature of the GPCRs allows them to regulate a wide variety of responses based off the Gα subunits, combined with the ability of multiple S1PRs to differentially couple to individual Gα subunits (

Table 1. S1P receptor subtypes.

Receptor	Gα Protein Subunit	Cell Types in CNS
S1PR1	Gαi	Astrocytes, microglia, neurons, and oligodendrocytes [24,25,26]
S1PR2	Gαi, Gαq/11, Gα12/13	Astrocytes, endothelial cells, microglia, and neurons [24,27,28]
S1PR3	Gαi, Gαq/11, Gα12/13	Astrocytes, microglia, neurons, and oligodendrocytes [29,30,31]
S1PR4	Gαi, Gα12/13	Astrocytes, endothelial cells, microglia, and neurons [24,32,33]
S1PR5	Gαi, Gα12/13	Endothelial cells and oligodendrocytes [24,34]

CNS: central nervous system; S1P: sphingosine-1-phosphate; S1PR: sphingosine-1-phosphate receptor.

), allowing for a diverse and robust response to S1P in the CNS.

4. Oligodendrocytes

Oligodendrocytes (OLGs) are the cells responsible for the myelination of neurons in the CNS. S1P receptor expression in OLGs has been found to be dependent on the maturity of the cell. S1PR1 and S1PR5 are both expressed in oligodendrocyte progenitor cells (OPCs) and continue to be detected throughout maturation into mature myelinating OLGs [35]. Interestingly, while OLG S1PR5 expression is higher than S1PR1 expression and S1PR5-deficient mature oligodendrocytes show an impaired response to S1P stimulation, S1PR5 knockout animals have no defects in myelination [36]. It has been proposed that this is due to compensation by S1PR1, and recent work has found that S1PR1 and S1PR5 are structurally similar, with conserved lipid-binding domains [37]. Further supporting this hypothesis is the ability of both S1PR1 and S1PR5 to couple with Gαi subunits (Table 1), as well as observations that responses to S1P mediated through both S1PR1 and S1PR5 confer pro-survival benefits to both OPCs and mature myelinating OLGs [24]. In addition to these findings, a comprehensive study was conducted on S1PR2 inactivation using adult male C57Bl/6 mice to better understand the receptor’s role in multiple sclerosis. These findings showcase the pivotal physiological roles of the receptor by demonstrating that inactivation decreases the number of macrophages, increases the number of oligodendrocytes in a lesion area, enhances the number of remyelinated axons, and increases oligodendrocyte progenitor cell (OPC) differentiation, with no effect on proliferation or recruitment [38].

5. Neurons

Neurons express S1PR1, S1PR2, and S1PR3 (Table 1), and their activation by S1P can often act in opposition to mediate neuronal function. This differential signaling is dependent on the Gα protein associated with the S1PR, as, similar to S1PR1, both S1PR2 and S1PR3 are able to couple to Gαi. In mature neurons, S1PR1 signaling provides protection against apoptosis via reductions in the expression of pro-apoptotic proteins BAX and HRK through activation of the Akt pathway [39]. S1PR1 also plays a role in sensory neurons, with evidence suggesting that roughly half of small-diameter sensory neurons express S1PR1 and that activation of these neurons via S1P increases their excitability [40]. Additionally, S1PR1 activation stimulates Rac and contributes to neuronal elongation [41], whereas the activation of S1PR2 and S1PR3 stimulates Rho and inhibits Rac, decreasing cell motility [42]. Interestingly, unlike S1PR2-5, mice deficient in S1PR1 expression suffer from impaired neural tube formation, contributing to embryonic death, suggesting a vital role for S1PR1 in neurodevelopment [43]. Further strengthening the role of S1PR1 in neurodevelopment are findings showing S1PR1 expression as early as E15 in mice at sites of active neurogenesis [44]. In neural progenitor cells (NPCs), inhibition of S1PR2 but not S1PR1 has been shown to inhibit migration to the site of ischemic injury, demonstrating that S1PR1-mediated migration of neuronal cells is conserved into adulthood and that this affect is opposed by S1PR2 [45]. Uniquely, S1PR2 can also be activated by interactions with the extracellular domain of NogoA and induce RhoA activity through the Gα13 subunit, although this interaction occurs outside the S1P binding domain [28]. NogoA-mediated activation of S1PR2 results in inhibition of neurite outgrowth, cell spreading, and reductions in synaptic plasticity similar to S1P-mediated S1PR2 activation, providing evidence that the two ligands converge on the same signal transduction pathways [28].

Similar to its role in ischemic injury repair, S1PR1 has also been shown to enhance neurogenesis and restore cognitive function in the hippocampus after traumatic brain injury via activation of the MEK/ERK pathway [46]. The association of S1P signaling and the MEK/ERK pathway highlights another important role of S1P/S1PR signaling in the amplification of receptor tyrosine kinase signals (Figure 2). The activation of most RTKs results in the activation of the mitogen-activated protein kinase (MAPK) cascade consisting of Ras, Raf, MEK, and ERK [47]. The translocation of SphK1 to the plasma membrane after RTK signaling, as discussed above, is dependent on the ERK1/2 phosphorylation of SphK1 at Ser 225 [48]. Once activated, SphK1 generates S1P, which can then be transported outside the cell to activate the S1PRs in both an autocrine and paracrine fashion [49]. Completing the positive feedback loop, activation of Gαi-coupled S1PRs can then activate the MAPK pathway, reinforcing SphK1 activation and enhancing the production of S1P. This provides a mechanism by which the body can elicit a robust mitogen signaling response without the need to produce large amounts of proteins, and due to the sphingolipid salvage pathway, the sphingosine used to generate S1P can also be recovered via dephosphorylation, further limiting biosynthetic needs within the cell. A classic example of this is nerve growth factor (NGF) signaling (Figure 1) mediated through the tropomyosin receptor kinase A (TrkA) receptor [50].

While the expression and function of S1PR1-3 have been well established and characterized in neurons, the detection and characterization of S1PR4 and S1PR5 remain controversial. While there are limited reports of synaptic expression of S1PR4 and S1PR5 in rat and mouse neurons, more work needs to be done to confirm these findings [51,52].

6. Microglia

The primary innate immune cells in the CNS are specialized macrophages called microglia. Similar to other organ-specific resident macrophages, microglia are professional phagocytic cells that surveil their tissue environment and eliminate pathogens and cellular debris [53]. Traditionally, microglia have been described as either resting or activated, with activated microglia being divided into two distinct categories referred to as M1 and M2. M1 microglia stimulate inflammation and subsequent neurotoxicity, whereas M2 microglia promote neuroprotection via anti-inflammatory effects [54]. With the advent of single-cell RNA sequencing (ssRNAseq), single-cell mass cytometry, and increased capabilities in live cell imaging, it has become apparent that this may have been an oversimplification that is no more useful than the body mass index as a measure of obesity [55,56]. Unfortunately, due to the recent discovery of this heterogeneity and the inertia behind the use of M1 and M2, much of the knowledge on S1PR signaling available for this review still relies on this flawed classification. As such, we will describe the roles of microglial S1PR signaling including these classifications despite the inherent flaws while also acknowledging the limitations.

Microglia express S1PR1, S1PR2, and S1PR3 isoforms, although most evidence suggests that all three receptors have conserved biological effects after stimulation. In mice, exposure to the amyloid β (Aβ) peptide results in an inflammatory microglial phenotype (or M1) characterized by the increased expression of toll-like receptor 4 (TLR4) and S1PR1, and isoform-specific S1PR1 inhibition prevents this activation by reducing Aβ-induced TLR4/S1PR1 expression increases and decreasing the levels of the proinflammatory cytokine TNF-α and chemokine CXCL10 [57]. Additionally, inhibition or siRNA-mediated knockdown of S1PR1 reduces microglial activation, TNF-α, and IL-1β expression after ischemia and lipopolysaccharide stimulation [58]. Similarly, in the context of diabetes-induced cognitive deficit in mice, inhibition of S1PR2 improves synaptic plasticity, decreases microglial apoptosis, reduces the levels of proinflammatory microglia, and increases the levels of anti-inflammatory microglia, traditionally described as reprogramming from an M1 to an M2 phenotype [59]. In the context of hepatic encephalopathy-induced hyperammonemia, S1PR2 activation contributes to astrocyte-mediated hippocampal neuroinflammation, IL-1β production, and cognitive impairment through the Gβγ-associated signaling cascade [60]. S1PR3 activation after cerebral ischemia has also been shown to contribute to the development of an M1 phenotype via ERK1/2 activation, increased microglial proliferation, and the production of proinflammatory cytokines, which could be reduced through the pharmacological inhibition of S1PR3 [61]. Conversely, activation of S1PR1 during ischemia promotes an anti-inflammatory M2 phenotype by reducing ERK1/2 activation and increasing Akt phosphorylation, suggesting a receptor-specific response to restricted blood flow to the brain and providing the only evidence of an anti-inflammatory (M2) response in microglia [58].

7. Astrocytes

Astrocytes are the most abundant cells in the brain, and these glial cells play multiple roles in the CNS, dependent on the specific brain region in which they reside. Due to this spatially based diversity in function, the morphology, cell surface receptor expression and production of cytokines and chemokines also vary based on their location [62]. Astrocytes can act in opposing roles, both enhancing immune responses and inhibiting myelin repair, but also dampen immune responses and support OG-mediated myelin repair [63]. Astrocytes also play roles in the establishment of the blood–brain barrier, regulate brain ion homeostasis, and modulate neuronal circuit function and assembly [64]. Due to their diverse range of function, astrocytes are a complex group of cells, and characterization and elucidation of the signaling pathways are harder than for other resident CNS cells.

S1PR1 expression is induced at astrocyte–neuron contact sites, and this expression drives complexity and induces the expression of SPARCL1 and thrombospondin 4, which are involved in synapse formation [65]. Loss of S1PR1 expression has been found to reduce astrocyte process extension and retraction, and the deletion of the gene in both Drosophila and zebrafish results in extensive impairment of motor function [66]. Additionally, the astrocyte-specific deletion of S1PR1 or its pharmacological inhibition has a protective effect in the context of the experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis, reducing the number of reactive astrocytes and preventing astrogliosis [67]. S1PR3 is highly expressed in astrocytes, and its expression increases in response to neuroinflammation. Stimulation of S1PR3 and, to a lesser extent, S1PR2, when coupled to Gα12/13, activates RhoA and potentiates inflammation by upregulating the expression of IL-6, COX-2, and VEGFa [68].

8. Endothelial Cells and Pericytes

Endothelial cells and pericytes form the core cellular constituents of the neurovascular unit and are essential for regulating cerebral blood flow, immune cell trafficking, and blood–brain barrier (BBB) integrity. Sphingosine-1-phosphate (S1P) signaling has emerged as a dominant regulator of vascular stability in the central nervous system, with sphingosine-1-phosphate receptors (S1PRs) coordinating cytoskeletal organization, junctional assembly, and inflammatory responses in both endothelial cells and pericytes.

Cerebral endothelial cells predominantly express S1PR1 and S1PR3, with S1PR1 serving as the principal mediator of barrier-stabilizing signaling under physiological conditions. Activation of endothelial S1PR1 promotes Rac1-dependent cortical actin remodeling; enhances adherens junction assembly; and stabilizes tight junction complexes, including claudin-5, occludin, and ZO-1, thereby limiting paracellular permeability [69]. In vivo studies have demonstrated that endothelium-specific deletion of S1PR1 results in increased vascular leakage and compromised BBB integrity, confirming an essential role for this receptor in maintaining cerebrovascular homeostasis [70].

Beyond structural stabilization, endothelial S1PR1 signaling suppresses inflammatory activation by limiting leukocyte adhesion and transendothelial migration. S1PR1 activation reduces the expression of adhesion molecules such as ICAM-1 and VCAM-1 and constrains NF-κB-dependent transcriptional programs in endothelial cells, thereby attenuating immune cell recruitment during neuroinflammatory and ischemic insults [71,72].

In contrast, S1PR3 signaling in endothelial cells is generally associated with vascular activation and increased permeability. S1PR3 couples to Gα12/13 and Gαq signaling pathways, promoting RhoA activation, actomyosin contraction, and junctional destabilization. Under conditions of inflammation, ischemia, or oxidative stress, upregulation of S1PR3 exacerbates endothelial dysfunction and BBB breakdown, contributing to leukocyte infiltration and neuroinflammatory amplification [72,73].

Pericytes, which are embedded within the microvascular basement membrane, play indispensable roles in BBB maturation, vascular stability, and regulation of capillary tone. Loss or dysfunction of pericytes leads to BBB disruption, neuroinflammation, and progressive neurodegeneration, underscoring their importance within the neurovascular unit [74,75]. Although pericyte-specific S1PR expression patterns are less comprehensively defined than those of endothelial cells, available evidence supports functional expression of S1PR1 and S1PR2, with S1P signaling influencing pericyte survival, contractility, and endothelial–pericyte crosstalk [76].

S1PR2 signaling in vasculature-associated cells has been linked to increased contractility and inflammatory signaling through Gα12/13–RhoA pathways, suggesting that excessive S1PR2 activation may contribute to microvascular dysfunction and BBB instability under pathological conditions. Consistent with this model, pharmacological inhibition of S1PR2 has been shown to preserve vascular integrity and reduce neuroinflammatory signaling in models of CNS injury and disease [77].

Collectively, S1P receptor signaling in endothelial cells and pericytes constitutes a central regulatory axis governing BBB integrity and neurovascular stability. The balance between barrier-stabilizing S1PR1 signaling and barrier-disruptive S1PR3 and S1PR2 pathways provides a mechanistic framework for understanding how vascular dysfunction, immune infiltration, and neuroinflammation emerge across diverse CNS pathologies. This cellular context underlies the barrier-specific roles of S1PR signaling discussed in the subsequent section and underscores the intersection between cardiovascular signaling pathways and CNS pathology.

9. S1PRs and CNS Barrier Function

The brain microenvironment is tightly regulated to ensure proper metabolic activity, limit exposure to blood pathogens, and promote proper neuronal function. Three separate barrier systems regulate the influx and efflux of molecules to and from the CNS, each serving distinct roles to protect the brain parenchyma. The most well characterized is the blood–brain barrier (BBB), which comprises a collection of astrocytes, microglia, endothelial cells, and pericytes in the capillary beds that supply the brain [71]. The BBB is the largest of the three barrier systems, with a combined surface area ranging between 12 and 18 m² in an average adult human [78]. The next most characterized system is the blood–cerebrospinal fluid barrier (BCSFB), mainly comprising CSF-facing epithelial cells in the choroid plexus that secrete CSF into the ventricular system in the brain [79]. Finally, tight junction-expressing epithelial-like cells in the meninges form the arachnoid barrier (AB), which isolates extracellular fluid in the CNS from the rest of the body [80]. The AB is the least characterized of the barrier systems, and its avascular nature results in limited impact on the influx of molecules into the CNS [78].

Of the three, S1P-S1PR signaling is best characterized in the BBB, with S1PR1 and S1PR3 working together to promote proper barrier function. S1PR1 signaling in in endothelial cells activates the Rho/Rac pathway, inducing cell spreading and reducing gaps between neighboring cells [81]. Additionally, S1PR1 activation promotes the expression and subcellular translocation of tight junction (TJ) proteins, including claudin-5, occluding, and ZO-1, stabilizing TJs [70]. Moreover, S1PR1 signaling also increases VE-cadherin translocation to sites of adherens junctions (AJs) in cell contact regions, further stabilizing cell-to-cell contacts [69]. S1PR3 signaling cells promote the migration and proliferation of endothelial progenitor cells and promote vasoconstriction in vascular smooth muscle cells, contributing to barrier function [72]. Interestingly, the loss of S1PR1 expression in mice results in a reversible increase in BBB permeability, suggesting that S1PR1 inhibition may provide a pharmacological strategy to increase the delivery of drugs into the CNS [70].

10. S1P Receptors in Stroke and Cerebral Ischemia

Ischemic stroke represents one of the most extensively investigated pathological contexts for sphingosine-1-phosphate (S1P) receptor signaling in the central nervous system, owing to the intimate coupling between cerebrovascular integrity, immune cell trafficking, glial activation, and neuronal survival. Dysregulation of the S1P–S1PR axis has been documented across multiple experimental models of cerebral ischemia, including transient and permanent middle cerebral artery occlusion (MCAO), global ischemia, and ischemia–reperfusion injury.

Among the five S1P receptor subtypes, S1PR1 has been most consistently implicated in ischemic pathophysiology. In models of transient focal ischemia, increased S1PR1 expression is observed in endothelial cells, microglia, and infiltrating immune cells following injury. Pharmacological inhibition or genetic attenuation of S1PR1 signaling reduces infarct volume, suppresses pro-inflammatory cytokine production, limits leukocyte infiltration, and improves neurological outcomes, supporting a pathogenic role for sustained S1PR1 activation during the acute post-ischemic phase [61]. Consistent with these findings, S1PR1 antagonism has been shown to reduce Toll-like receptor 4 (TLR4)-dependent inflammatory signaling in ischemic and amyloid-exposed brains, further linking S1PR1 to innate immune amplification after injury [82].

In parallel, S1PR1 signaling plays a critical role in regulating blood–brain barrier (BBB) integrity following ischemia. Endothelial S1PR1 activity promotes tight junction stability and vascular barrier function under physiological conditions; however, experimental modulation of S1PR1 has demonstrated that transient receptor inhibition can induce size-selective BBB opening without causing overt vascular damage. This observation has generated interest in S1PR1 as a dual-function target capable of both exacerbating ischemic inflammation and enabling controlled therapeutic access to the injured brain [70].

In contrast to S1PR1, S1PR2 signaling is generally associated with inhibition of reparative and migratory responses after ischemic injury. Antagonism of S1PR2 enhances neural progenitor cell migration toward ischemic lesions and improves tissue repopulation in rodent stroke models, implicating S1PR2 as a negative regulator of endogenous repair mechanisms [45]. At the molecular level, S1PR2 couples to Gα12/13 and RhoA signaling pathways, which restrict cytoskeletal dynamics and cell motility, providing a mechanistic basis for its inhibitory effects on regeneration.

S1PR3 has also emerged as an important contributor to ischemia-induced neuroinflammation. Studies in murine models of cerebral ischemia have demonstrated that S1PR3 activation promotes microglial proliferation, ERK1/2 activation, and polarization toward a pro-inflammatory phenotype. Genetic deletion or pharmacological inhibition of S1PR3 attenuates microglial activation, reduces cytokine production, and limits ischemic brain injury, identifying S1PR3 as a driver of inflammatory amplification following stroke [61].

Beyond receptor-specific effects, pharmacological modulation of the S1P–S1PR axis has demonstrated robust efficacy in preclinical stroke models. Fingolimod (FTY720), a functional antagonist of S1PR1, S1PR3, S1PR4, and S1PR5, reduces infarct volume, suppresses neuroinflammation, and improves neurological recovery when administered after ischemic injury. These protective effects have been attributed to reduced lymphocyte trafficking, stabilization of the BBB, and attenuation of microglial activation [83,84].

Importantly, early clinical studies have suggested that fingolimod may confer benefit in human ischemic stroke, with reported reductions in infarct growth, improved neurological outcomes, and decreased hemorrhagic transformation when administered in combination with thrombolytic therapy [85]. While these studies remain limited in scale, they underscore the translational relevance of S1PR modulation in cerebrovascular disease.

Collectively, the available literature supports a model in which S1P receptors exert receptor-specific, cell type-dependent, and temporally distinct effects during ischemic stroke. S1PR1 and S1PR3 predominantly promote inflammatory and vascular dysfunction in the acute phase, whereas S1PR2 restricts regenerative responses, positioning the S1P–S1PR axis as a complex but highly actionable therapeutic target in ischemic brain injury.

11. S1P Receptors in Neurodegenerative Disease

Dysregulation of sphingosine-1-phosphate (S1P) signaling has been increasingly implicated in the pathogenesis of multiple neurodegenerative disorders, reflecting the central role of S1P receptors (S1PRs) in regulating neuroinflammation, glial activation, synaptic function, and neuronal survival (Table 2). Altered expression of S1P receptors, changes in sphingosine kinase activity, and disruption of S1P gradients have been reported across Alzheimer’s disease (AD), Parkinson’s disease (PD), and amyotrophic lateral sclerosis (ALS), positioning the S1P–S1PR axis as a shared mechanistic pathway in neurodegeneration.

11.1. Alzheimer’s Disease

In Alzheimer’s disease, multiple lines of evidence support roles for S1PR signaling in amyloid pathology, neuroinflammation, and synaptic dysfunction. Postmortem analyses and experimental models demonstrate altered expression of S1PR1 and S1PR2 in the AD brain, particularly in astrocytes and microglia associated with amyloid plaques. Pharmacological inhibition of S1PR1 reduces microglial activation, suppresses pro-inflammatory cytokine production, and enhances amyloid-β (Aβ) clearance in transgenic mouse models of AD, indicating that sustained S1PR1 signaling contributes to inflammatory amplification rather than neuroprotection in this context [57].

Consistent with these findings, modulation of S1PR signaling using fingolimod (FTY720) attenuates synaptic loss, reduces neuroinflammation, and improves cognitive performance in multiple AD mouse models. These effects are associated with decreased microglial reactivity, normalization of astrocytic function, and restoration of synaptic protein expression, suggesting that S1PR modulation acts at the level of glial–neuronal crosstalk rather than directly targeting amyloid production [86,87].

11.2. Parkinson’s Disease

In Parkinson’s disease, S1P receptor signaling has been linked to dopaminergic neuron survival, mitochondrial integrity, and neuroinflammatory responses. Experimental models of PD demonstrate that activation of S1PR1promotes neuronal survival through PI3K/Akt and ERK signaling pathways, while pharmacological inhibition of sphingosine kinase or S1PR signaling exacerbates dopaminergic neuron loss [39].

Treatment with fingolimod or related S1PR modulators confers neuroprotection in toxin-based models of PD, reducing dopaminergic cell loss, suppressing microglial activation, and improving motor performance. These effects are attributed to both direct neuronal signaling and indirect anti-inflammatory actions, highlighting the dual cell-autonomous and non-cell-autonomous roles of S1PRs in PD pathology [88,89].

11.3. Amyotrophic Lateral Sclerosis

Emerging evidence also implicates S1P receptor signaling in amyotrophic lateral sclerosis. In ALS models, altered sphingosine kinase activity and reduced S1P availability are associated with motor neuron vulnerability and glial dysfunction. Pharmacological modulation of S1PRs using fingolimod delays disease onset, reduces motor neuron loss, and extends survival in SOD1-mutant mouse models of ALS, effects that are accompanied by attenuation of microglial activation and reduced astrocytic toxicity [90].

Notably, early-phase clinical studies have demonstrated that fingolimod is safe and well tolerated in ALS patients, although clear efficacy signals remain limited. These findings nonetheless support continued investigation of S1PR modulation as a disease-modifying strategy in ALS, particularly in combination with therapies targeting complementary pathogenic pathways [91].

11.4. Multiple Sclerosis

Multiple sclerosis (MS) represents the prototypical clinical success of S1P receptor modulation and provides the clearest demonstration of the translational relevance of S1PR biology in the central nervous system. MS is characterized by immune-mediated demyelination, axonal injury, and progressive neurodegeneration driven by dysregulated lymphocyte trafficking and glial activation. Functional antagonism of S1PR1 reduces egress of autoreactive lymphocytes from secondary lymphoid organs, thereby limiting CNS infiltration and attenuating inflammatory demyelination [33,92]. Beyond peripheral immune sequestration, accumulating evidence indicates that S1PR modulation exerts direct CNS effects, including reduced astrocyte activation, modulation of microglial inflammatory signaling, and support of oligodendrocyte survival and remyelination via S1PR5 signaling [24,38,93].

Clinically approved S1PR modulators—including fingolimod, siponimod, ozanimod, and ponesimod—demonstrate reduced relapse rates, decreased lesion formation as observed by MRI, and delayed disability progression in relapsing forms of MS, with siponimod additionally showing efficacy in secondary progressive MS [92,94,95]. These outcomes validate the S1P–S1PR axis as a disease-modifying therapeutic target and establish MS as a foundational model for receptor-selective modulation in CNS inflammatory disease.

11.5. Convergent Mechanisms Across Neurodegenerative Diseases

Across AD, PD, and ALS, a unifying theme is the involvement of S1P receptors in regulating neuroinflammation, glial–neuronal communication, and cellular stress responses. While receptor-specific roles vary by disease context, sustained activation of S1PR1 and S1PR3 is frequently associated with inflammatory amplification, whereas therapeutic modulation of the S1P–S1PR axis dampens pathological signaling and promotes functional recovery. These shared mechanisms underscore the relevance of S1PRs as convergent modulators of neurodegeneration and reinforce their potential as broadly applicable therapeutic targets.

Table 2. S1P receptor involvement in central nervous system diseases.

CNS Disease	Major Findings
Ischemic Stroke/Cerebral Ischemia	S1PR1 and S1PR3 promote post-ischemic inflammation and BBB dysfunction [34,96]; S1PR2 restricts neural progenitor migration and repair [45,97]; pharmacological S1PR modulation reduces infarct size and improves neurological outcome [34,45,96].
Alzheimer’s Disease	S1PR1 signaling promotes neuroinflammation and impairs amyloid clearance, and S1PR modulation reduces glial activation, synaptic loss, and cognitive decline in AD models [57,96,98].
Parkinson’s Disease	S1PR1 signaling supports the survival of dopaminergic neurons via Akt/ERK pathways; S1PR modulation attenuates neuroinflammation and motor deficits in PD models [88,89,96,99,100].
Amyotrophic Lateral Sclerosis	Altered S1P signaling contributes to motor neuron vulnerability; S1PR modulation delays disease progression and reduces neuroinflammation in ALS models; early clinical studies demonstrate safety [25,101,102,103,104].
Multiple Sclerosis	Functional antagonism of S1PR1 reduces immune cell trafficking and neuroinflammation; S1PR5 signaling supports oligodendrocyte survival and myelination [24,25,57,93,105]

Summary of sphingosine-1-phosphate receptor (S1PR) involvement in major central nervous system diseases. This table highlights receptor-specific and disease-dependent roles of S1PR signaling in neuroinflammation, neurodegeneration, and cerebrovascular dysfunction. AD: Alzheimer’s Disease; ALS: Amyotrophic Lateral Sclerosis; BBB: Blood–Brain Barrier; S1PR: Sphingosine-1-phosphate receptor.

Table 2. S1P receptor involvement in central nervous system diseases.

CNS Disease	Major Findings
Ischemic Stroke/Cerebral Ischemia	S1PR1 and S1PR3 promote post-ischemic inflammation and BBB dysfunction [34,96]; S1PR2 restricts neural progenitor migration and repair [45,97]; pharmacological S1PR modulation reduces infarct size and improves neurological outcome [34,45,96].
Alzheimer’s Disease	S1PR1 signaling promotes neuroinflammation and impairs amyloid clearance, and S1PR modulation reduces glial activation, synaptic loss, and cognitive decline in AD models [57,96,98].
Parkinson’s Disease	S1PR1 signaling supports the survival of dopaminergic neurons via Akt/ERK pathways; S1PR modulation attenuates neuroinflammation and motor deficits in PD models [88,89,96,99,100].
Amyotrophic Lateral Sclerosis	Altered S1P signaling contributes to motor neuron vulnerability; S1PR modulation delays disease progression and reduces neuroinflammation in ALS models; early clinical studies demonstrate safety [25,101,102,103,104].
Multiple Sclerosis	Functional antagonism of S1PR1 reduces immune cell trafficking and neuroinflammation; S1PR5 signaling supports oligodendrocyte survival and myelination [24,25,57,93,105]

Summary of sphingosine-1-phosphate receptor (S1PR) involvement in major central nervous system diseases. This table highlights receptor-specific and disease-dependent roles of S1PR signaling in neuroinflammation, neurodegeneration, and cerebrovascular dysfunction. AD: Alzheimer’s Disease; ALS: Amyotrophic Lateral Sclerosis; BBB: Blood–Brain Barrier; S1PR: Sphingosine-1-phosphate receptor.

12. S1PR Modulation

Due to the ubiquitous involvement of S1P signaling in neuroprotection, modulation of S1PR activity has gained interest in the treatment of multiple diseases. The first S1PR modulator approved by the FDA was fingolimod, which has been used to treat relapsing and remitting multiple sclerosis (RRMS) since 2010 [92]. To date, more than 30 synthetic small molecules have been developed to modulate the activity of the S1PRs, with many being synthetic sphingosine derivatives [106]. Physiologically, S1P receptors 1–3 are expressed in all organ systems across the body, with S1PR 5 being expressed mainly in the immune system and S1PR5 expression mostly limited to oligodendrocytes in the CNS and natural killer cells in the immune system [107]. As previously discussed, S1P receptors bind to a diverse array of Gα proteins and can act to either in conjunction with or in opposition to each other. Due to this, efforts have been made to develop and characterize both isoform-specific agonists (

Table 3. S1P receptor agonists.

Name	Receptor	CAS Number
AKP 11	S1PR1	1220973-37-4
APD 334	S1PR1	1206123-37-6
CS 2100	S1PR1	913827-99-3
CS-0777	S1PR1	840523-39-9
CYM 5442	S1PR1	1783987-80-3
KRP-203	S1PR1	509088-69-1
RP 001	S1PR1	1781880-34-9
SEW 2871	S1PR1	256414-75-2
Sy 1930	S1PR1	1418093-75-0
AMG-369	S1PR1/S1PR5	1202073-26-4
GSK 2018682	S1PR1/S1PR5	1034688-30-6
CYM 5520	S1PR2	1449747-00-5
CYM 5541	S1PR3	945128-26-7
CYM 50260	S1PR4	1355026-60-6
CYM 50308	S1PR4	1345858-76-5
A 971432	S1PR5	1240308-45-5

S1P: sphingosine-1-phosphate; S1PR: sphingosine-1-phosphate receptor; CAS: Chemical Abstracts Service Registry.

) and antagonists (

Table 4. S1P receptor antagonists.

Name	Receptor	CAS Number
AD2900	pan	18360-29-7
Ex 26	S1PR1	1233332-37-0
KSI-6666	S1PR1	1807873-14-8
TASP 0277308	S1PR1	945725-50-8
W146	S1PR1	909725-61-7
VPC 23019	S1PR1/S1PR3	449173-19-7
JTE-013	S1PR2/S1PR4	383150-41-2
TY 52156	S1PR3	934369-14-9
CYM 50358	S1PR4	1781750-72-8

) to study their discrete functions and explore the potential for disease-modifying treatments (DMTs) [106]. Despite the number of small molecules developed to activate or inhibit the activity of S1PRs, none of these molecules has been approved for the treatment of any pathological condition.

A third class of S1PR modulators, the members of which exert their effects through functional antagonism (

Table 5. S1P receptor functional antagonists.

Name	Receptor	CAS Number
Fingolimod * (FTY720)	S1PR1/S1PR3/S1PR4/S1PR5	162359-55-9
Amiselimod *	S1PR1/S1PR4/S1PR5	942398-84-7
Ceralifimod	S1PR1/S1PR5	891859-12-4
Etrasimod	S1PR1/S1PR4/S1PR5	1206123-37-6
Ozanimod	S1PR1/S1PR5	1306760-87-1
Ponesimod	S1PR1	854107-55-4
Siponimod	S1PR1/S1PR5	1230487-00-9
ST-1478 *	S1PR1/S1PR5	# [110]
ST-1505	S1PR1/S1PR3	# [110]

S1P: sphingosine-1-phosphate; S1PR: sphingosine-1-phosphate receptor; CAS: Chemical Abstracts Service Registry; IUPAC: International Union of Pure Applied Chemistry. * prodrugs that require phosphorylation by SphK2 for activation. # Note: ST-1478 and ST-1505 are not registered with the Chemical Abstracts Service Registry; however, their IUPAC standard names are 2-amino-2-(2-{4-[3-(piperidin-1-yl)propoxy]phenyl}ethyl)propane-1,3-diol and 7a-[2-[4-(3-piperidin-1-ylpropoxy)phenyl]ethyl]-1,3,5,7-tetrahydro-[1,3]oxazolo [3,4-c][1,3]oxazole, respectively.

), has found greater success. These drugs exert their antagonistic effects through initial activation of the S1PR, followed by decreased expression on the cell surface, resulting in desensitization of the cell to subsequent S1P-induced signaling [108]. Fingolimod (Gilenya^®, Novartis, East Hanover, NJ, USA) the first approved S1PR modulator, shows higher affinity to S1PR1 but is able to bind to and interact with all S1PRs, except S1PR2, increasing the risk of off-target effects [109]. Attempts to increase receptor selectivity have given rise to a second generation of functional antagonists, some of which have found success in preclinical studies or clinical use both inside and outside the CNS. Of particular note are ozanimod (Zeposia^®, Bristol Myers Squibb, Princeton, NJ, USA), ponesimod (Ponvory^®, Janssen Pharmaceuticals, Titusville, NJ, USA), siponimod (Mayzent^®, Novartis), and etrasimod (Velsipity^®, Pfizer, New York, NY, USA), which have all received FDA approval within the past 5 years. Ozanimod, which targets S1PR1 and S1PR5, received FDA approval in 2020 for the treatment of MS and 2021 for the treatment of ulcerative colitis and is currently undergoing phase III trials for the treatment of Chron’s disease [94]. Ponesimod, which targets S1PR1, received FDA approval in 2023 for the treatment of MS and has completed phase II trials examining its potential for the treatment of psoriasis [95]. Siponimod, which targets S1PR1 and S1PR5, was approved in 2019 for the treatment of MS and has just begun a phase II clinical trial to examine its potential for the treatment of Alzheimer’s disease (NCT06639282). Etrasimod, which targets S1PR1, S1PR4, and S1PR5, received FDA approval in 2023 for the treatment of MS and is undergoing phase II trials for the treatment of immune checkpoint inhibitor diarrhea (NCT06521762) and ulcerative colitis (NCT06398626) and a phase II/III trial for the treatment of Chron’s disease (NCT04173273).

13. Structural Basis of S1P Receptor Selectivity and Modulation

Recent advances in X-ray crystallography and cryo-electron microscopy have provided detailed structural insight into sphingosine-1-phosphate receptor (S1PR) activation, ligand recognition, and subtype selectivity, substantially refining mechanistic models of S1P signaling. High-resolution structures of S1PR1, S1PR3, and S1PR5 in complex with endogenous ligands, synthetic modulators, and heterotrimeric G proteins have revealed both conserved architectural features and receptor-specific determinants that underlie signaling bias, pharmacological selectivity, and functional antagonism.

All S1PRs adopt the canonical seven-transmembrane (7TM) fold characteristic of class A G protein-coupled receptors (GPCRs), with a deeply embedded ligand-binding pocket adapted for amphipathic lysophospholipids. Structural analyses have demonstrated that the charged phosphate headgroup of S1P interacts with conserved basic residues near the extracellular face of the receptor, while the hydrophobic sphingoid tail extends into a lipid-accessible channel formed primarily by transmembrane helices III, V, and VI [11,111]. This ligand entry mode distinguishes S1PRs from peptide-binding GPCRs and explains their sensitivity to membrane lipid composition.

Despite overall structural conservation, subtype-specific differences in extracellular loop architecture and transmembrane helix orientation confer ligand selectivity and signaling diversity. Cryo-EM structures of S1PR1 and S1PR5 reveal divergence in extracellular loop 2 and the upper region of transmembrane helix V, altering ligand access and stabilizing distinct active conformations [37]. These features provide a structural rationale for the preferential targeting of S1PR1 and S1PR5 by second-generation modulators such as siponimod and ozanimod, as well as the relative pharmacological resistance of S1PR2, which exhibits a more constrained extracellular vestibule.

Structural characterization of S1PR3 further highlights receptor-specific determinants of signaling output. Comparison of S1PR1 and S1PR3 structures indicates differences in intracellular loop conformation and G protein-contacting residues that favor coupling of S1PR3 to Gαq and Gα12/13, in addition to Gαi. These features correlate with the enhanced pro-inflammatory and barrier-disruptive signaling associated with S1PR3 activation in endothelial cells and glia [42,72].

Structural studies have also clarified the mechanistic basis of functional antagonism, a defining property of several clinically approved S1PR modulators. Fingolimod phosphate and related ligands stabilize receptor conformations that favor β-arrestin recruitment and receptor internalization, leading to sustained desensitization despite initial agonist activity. Crystallographic and cryo-EM analyses have demonstrated that these ligands induce conformational states distinct from those stabilized by endogenous S1P, providing a structural explanation for prolonged receptor downregulation and lymphocyte sequestration [111,112].

More recent structures of S1PR5 have revealed additional mechanisms of ligand-dependent modulation, including inverse agonism. Structural analysis of S1PR5 bound to inverse agonists demonstrates stabilization of inactive receptor conformations that suppress basal signaling activity, a property that may contribute to improved therapeutic windows and reduced adverse effects [107]. These findings highlight how subtle structural differences between S1PR subtypes can be exploited to achieve refined pharmacological control.

Collectively, structural studies establish that S1PR signaling outcomes are encoded at the level of receptor architecture, with ligand chemistry selectively engaging subtype-specific features to bias downstream signaling pathways. Integration of structural and functional data has therefore become central to the rational design of next-generation S1PR modulators optimized for CNS efficacy while minimizing cardiovascular and immunological liabilities. These structural determinants explain not only subtype-selective ligand recognition but also why functional antagonists such as fingolimod and second-generation modulators exhibit distinct efficacy and adverse-effect profiles in CNS disease.

14. Adverse Effects and Behavioral Consequences of S1P Receptor Modulation

While pharmacological modulation of sphingosine-1-phosphate receptors (S1PRs) has demonstrated therapeutic efficacy across multiple central nervous system (CNS) disease models, both preclinical and clinical studies have revealed a range of adverse effects and behavioral consequences associated with sustained or systemic manipulation of S1P signaling. These effects reflect the broad physiological roles of S1PRs in vascular regulation, immune cell trafficking, and neural function.

14.1. Systemic and Neurological Adverse Effects

The most extensively characterized adverse effects of S1PR modulation arise from S1PR1 functional antagonism, particularly with first-generation agents such as fingolimod (FTY720). In clinical use, fingolimod is associated with transient bradycardia, atrioventricular conduction delays, and hypertension, effects attributed to S1PR1 and S1PR3 signaling in cardiac and vascular tissues [92,109]. These cardiovascular effects have motivated the development of second-generation S1PR modulators with increased receptor selectivity and improved safety profiles.

Neurological adverse effects have also been reported, including headache, dizziness, fatigue, and sleep disturbances, although severe CNS toxicity remains uncommon in approved dosing regimens. Importantly, prolonged S1PR modulation can increase susceptibility to infections due to impaired lymphocyte egress from secondary lymphoid organs, an effect that indirectly influences neuroinflammatory states and CNS immune surveillance [112,113].

Second-generation agents such as siponimod, ozanimod, and ponesimod exhibit reduced cardiac liability and improved pharmacokinetic control, though dose-dependent lymphopenia and hepatic enzyme elevations have still been observed, underscoring the need for careful titration and monitoring [114,115,116].

14.2. Behavioral Effects in Preclinical Models

In animal models, modulation of S1PR signaling has been associated with measurable behavioral outcomes, reflecting the involvement of S1P receptors in synaptic plasticity, neurogenesis, and glial–neuronal communication. In rodent studies, fingolimod treatment has been found to improve cognitive performance and spatial memory in models of Alzheimer’s disease and traumatic brain injury, effects correlated with reduced neuroinflammation and enhanced hippocampal neurogenesis [46,86].

Conversely, excessive or non-selective S1PR modulation has been reported to impair locomotor activity and induce anxiety-like behaviors in certain experimental paradigms, suggesting that disruption of physiological S1P gradients may negatively impact neuronal circuit function. These effects are highly context-dependent and appear to vary with receptor subtype specificity, dosing regimen, and duration of exposure [51,67,117].

14.3. Human Behavioral and Cognitive Outcomes

In clinical populations, the behavioral effects of S1PR modulators have been most extensively evaluated in multiple sclerosis, where treatment with fingolimod and second-generation agents has been associated with stabilization or modest improvement of cognitive function, fatigue, and quality-of-life measures. These benefits are thought to arise indirectly from reduced neuroinflammation rather than direct neuromodulatory effects [115,118,119].

Importantly, available clinical data do not support widespread adverse neuropsychiatric effects of approved S1PR modulators when administered within recommended dosing parameters. However, long-term behavioral consequences outside of inflammatory disease contexts remain incompletely characterized, particularly in neurodegenerative disorders, where baseline cognitive decline may confound assessment.

14.4. Implications for Therapeutic Development

Together, these findings highlight the necessity of balancing therapeutic efficacy with receptor selectivity, dosing strategy, and duration of S1PR modulation. Behavioral and adverse-effect profiles are strongly influenced by receptor subtype targeting and tissue distribution, reinforcing the rationale for next-generation S1PR modulators optimized for CNS efficacy while minimizing systemic exposure. As demonstrated in

Table 6. Dosing, receptor selectivity, and potency of clinically relevant S1PR modulators.

Compound	Primary S1PR Target(s)	Typical Clinical Dose (Route, Disease)	Representative In Vitro Potency
Fingolimod (FTY720)	S1PR1, S1PR3, S1PR4, S1PR5	0.5 mg/day (oral, MS)	EC50 (S1PR1): ~0.3–0.6 nM (after phosphorylation) [120,121]
Siponimod (BAF312)	S1PR1, S1PR5	2 mg/day (oral, SPMS)	EC50 (S1PR1): ~0.4 nM [121,122]
Ozanimod (RPC1063)	S1PR1, S1PR5	0.92 mg/day (oral, MS/UC)	EC50 (S1PR1): ~0.4 nM [121]
Ponesimod	S1PR1	20 mg/day (oral, MS)	EC50 (S1PR1): ~5–6 nM [121,123]
Etrasimod	S1PR1, S1PR4, S1PR5	2 mg/day (oral, UC)	EC50 (S1PR1): ~6–7 nM [121,124]

S1PR, sphingosine-1-phosphate receptor; MS: multiple sclerosis; SPMS: sporadic MS; UC: ulcerative colitis; EC₅₀: half maximal effective concentration.

, while all of the current clinically relevant S1PR modulators share commonality in route of delivery and the targeting of S1PR1, there are 10-fold differences in potency in terms of inhibiting S1PR1 activity between the most selective inhibitor (ponesimod) and the least selective inhibitor (fingolimod), despite a 40-fold difference in clinical dose for the management of MS. This highlights the need for continued integration of behavioral and neurological endpoints in both preclinical and clinical studies to refine the therapeutic potential of S1PR-targeted interventions.

15. Conclusions

This review highlights sphingosine-1-phosphate receptor (S1PR) signaling as a central regulatory axis in central nervous system physiology and pathology, integrating lipid metabolism with cell type-specific signaling, barrier function, and disease progression. Rather than acting through a uniform mechanism, S1PR signaling exhibits pronounced spatial, temporal, and cellular specificity, with distinct receptor subtypes mediating divergent outcomes across neurons, oligodendrocytes, astrocytes, microglia, endothelial cells, and pericytes. These context-dependent signaling properties help explain how a single bioactive lipid can influence processes as diverse as neurodevelopment, immune surveillance, vascular integrity, and neurodegeneration.

A key theme emerging from recent work is the importance of S1PR signaling at CNS interfaces, particularly within the neurovascular unit and blood–brain barrier. Endothelial and pericyte S1PR signaling coordinates vascular stability, immune cell trafficking, and barrier permeability, positioning this pathway as a mechanistic link between systemic physiology and CNS homeostasis. Dysregulation of these processes contributes not only to inflammatory demyelinating disease but also to stroke, neurodegenerative disorders, and age-associated cognitive decline, underscoring the broad relevance of S1PR biology beyond multiple sclerosis.

Advances in receptor-selective pharmacology have further clarified how differential engagement of S1PR subtypes can yield distinct therapeutic outcomes. The clinical success of S1PR modulators demonstrates the tractability of this pathway while also revealing limitations associated with non-selective or sustained receptor modulation. These findings emphasize the need for refined strategies that account for receptor subtype, cell-type specificity, signaling bias, and treatment timing. Parallel progress in S1PR structural biology now provides a foundation for the rational design of next-generation modulators with improved selectivity and safety profiles.

Looking forward, several priorities emerge. Greater resolution of spatiotemporal S1PR expression and signaling dynamics across CNS cell types will be essential for understanding disease stage-specific functions. The integration of in vivo models with human iPSC-derived systems offers a promising route to bridge species differences and identify conserved mechanisms. Finally, coupling functional outcomes with multi-omic and structural approaches will be critical for defining how S1PR signaling networks adapt in health and fail in disease. Together, these directions position the S1P–S1PR axis as a unifying framework for understanding CNS pathology and as a fertile target space for future therapeutic development.