Attenuation of cell motility observed with high doses of sphingosine 1-phosphate or phosphorylated FTY720 involves RGS2 through its interactions with the receptor S1P

Critical role of SP thymocyte motility in regulation of thymic output in neonatal Aire-/- mice

Autoimmune regulator (Aire) is essential in the perinatal period to prevent the multiorgan autoimmunity. Here we show that Aire-regulated single positive thymocyte trafficking in neonatal period is critical for thymic egress. Reduced thymic emigration was found in Aire-/- mice during neonatal period, leading to enhanced homeostatic expansion of peripheral T cells as early as 2 weeks of age. In neonatal Aire-/- mice, thymic expression of CCR7 ligands were dramatically reduced, resulting in decreased thymocyte motility and thymocyte emigration. This reduction of thymic egress in Aire-/- mice was alleviated beyond 3 weeks of age by an early upregulation of S1P1 signaling. As the numbers and quality of thymic emigrants are essential for the establishment and maintenance of peripheral tolerance, the reduced thymic emigration during neonatal period may deteriorate autoimmunity caused by the emigration of autoreactive T cells.

Sphingosine-1-phosphate/sphingosine-1-phosphate receptor 1 signaling is required for migration of naive human T cells from the thymus to the periphery

BACKGROUND

The mechanisms that govern the egress of mature thymocytes from the human thymus to the periphery remain understudied yet are of utmost importance to the field of basic immunology, as well as T-cell reconstitution in various immunodeficiencies. We examined the expression and function of sphingosine-1-phosphate (S1P) receptors in human thymocyte egress.

OBJECTIVES

We aimed to determine whether S1P receptors (S1P-Rs) play a role in mature human thymocyte egress and to identify the thymocyte population or populations that express S1P-Rs and respond to S1P by migrating across a concentration gradient.

METHODS

Human thymocytes were exposed to S1P in Transwell plate migration assays coupled to flow cytometry to evaluate the response to S1P of thymocytes at different stages of maturation. Constitutive S1P-R expression was quantified by means of real-time PCR in sorted thymocyte subsets and flow cytometry. S1P-R1 and Kruppel-like factor 2 expression were monitored after S1P exposure by using flow cytometry and quantitative PCR.

RESULTS

S1P-R1 was the prevalent S1P receptor on mature human thymocytes (CD3(hi)CD27(+)CD69(-)), the population that also demonstrated the greatest response to S1P in migration assays. Pretreatment with FTY720, an S1P-R1 nonselective modulator significantly reduced migration and suggested a role for S1P-R2 in retaining thymocytes in the tissue. Lastly, surface S1P-R1 expression, as well S1PR1 and Kruppel-like factor 2 (KLF2) transcripts, were significantly decreased in mature thymocytes on exposure to S1P.

CONCLUSION

Mature human thymocytes rely on S1P-R1 to migrate toward S1P. Taken in the context of murine work demonstrating that S1P is required for thymocyte egress to the periphery, our data highlight a new key chemokine for human thymocyte egress.

Fingolimod modulates multiple neuroinflammatory markers in a mouse model of Alzheimer's disease

Sphingosine 1-phosphate (SP1) receptors may be attractive targets for modulation of inflammatory processes in neurodegenerative diseases. Recently fingolimod, a functional S1P1 receptor antagonist, was introduced for treatment of multiple sclerosis. We postulated that anti-inflammatory mechanisms of fingolimod might also be protective in Alzheimer’s disease (AD). Therefore, we treated a mouse model of AD, the 5xFAD model, with two doses of fingolimod (1 and 5 mg/kg/day) and measured the response of numerous markers of Aβ pathology as well as inflammatory markers and neurochemistry using biochemical, immunohistochemistry and high resolution magic angle spinning magnetic resonance spectroscopy (MRS). In mice at 3 months of age, we found that fingolimod decreased plaque density as well as soluble plus insoluble Aβ measured by ELISA. Fingolimod also decreased GFAP staining and the number of activated microglia. Taurine has been demonstrated to play a role as an endogenous anti-inflammatory molecule. Taurine levels, measured using MRS, showed a very strong inverse correlation with GFAP levels and ELISA measurements of Aβ, but not with plaque density or activated microglia levels. MRS also showed an effect of fingolimod on glutamate levels. Fingolimod at 1 mg/kg/day provided better neuroprotection than 5 mg/kg/day. Together, these data suggest a potential therapeutic role for fingolimod in AD.

SphK1 mediates hepatic inflammation in a mouse model of NASH induced by high saturated fat feeding and initiates proinflammatory signaling in hepatocytes

Steatohepatitis occurs in up to 20% of patients with fatty liver disease and leads to its primary disease outcomes, including fibrosis, cirrhosis, and increased risk of hepatocellular carcinoma. Mechanisms that mediate this inflammation are of major interest. We previously showed that overload of saturated fatty acids, such as that which occurs with metabolic syndrome, induced sphingosine kinase 1 (SphK1), an enzyme that generates sphingosine-1-phosphate (S1P). While data suggest beneficial roles for S1P in some contexts, we hypothesized that it may promote hepatic inflammation in the context of obesity. Consistent with this, we observed 2-fold elevation of this enzyme in livers from humans with nonalcoholic fatty liver disease and also in mice with high saturated fat feeding, which recapitulated the human disease. Mice exhibited activation of NFκB, elevated cytokine production, and immune cell infiltration. Importantly, SphK1-null mice were protected from these outcomes. Studies in cultured cells demonstrated saturated fatty acid induction of SphK1 message, protein, and activity, and also a requirement of the enzyme for NFκB signaling and increased mRNA encoding TNFα and MCP1. Moreover, saturated fat-induced NFκB signaling and elevation of TNFα and MCP1 mRNA in HepG2 cells was blocked by targeted knockdown of S1P receptor 1, supporting a role for this lipid signaling pathway in inflammation in nonalcoholic fatty liver disease.

Amphiphilic degradable polymers for immobilization and sustained delivery of sphingosine 1-phosphate

Controlled delivery of the angiogenic factor sphingosine 1-phosphate (S1P) represents a promising strategy for promoting vascularization during tissue repair and regeneration. In this study, we developed an amphiphilic biodegradable polymer platform for the stable encapsulation and sustained release of S1P. Mimicking the interaction between amphiphilic S1P and its binding proteins, a series of polymers with hydrophilic poly(ethylene glycol) core and lipophilic flanking segments of polylactide and/or poly(alkylated lactide) with different alkyl chain lengths were synthesized. These polymers were electrospun into fibrous meshes, and loaded with S1P in generally high loading efficiencies (>90%). Sustained S1P release from these scaffolds could be tuned by adjusting the alkyl chain length, blockiness and lipophilic block length, achieving 35-55% and 45-80% accumulative releases in the first 8h and by 7 days, respectively. Furthermore, using endothelial cell tube formation assay and chicken chorioallantoic membrane assay, we showed that the different S1P loading doses and release kinetics translated into distinct pro-angiogenic outcomes. These results suggest that these amphiphilic polymers are effective delivery vehicles for S1P and may be explored as tissue engineering scaffolds where the delivery of lipophilic or amphiphilic bioactive factors is desired.

A finer tuning of G-protein signaling through regulated control of RGS proteins

Regulators of G-protein signaling (RGS) proteins are GTPase-activating proteins (GAP) for various Gα subunits of heterotrimeric G proteins. Through this mechanism, RGS proteins regulate the magnitude and duration of G-protein-coupled receptor signaling and are often referred to as fine tuners of G-protein signaling. Increasing evidence suggests that RGS proteins themselves are regulated through multiple mechanisms, which may provide an even finer tuning of G-protein signaling and crosstalk between G-protein-coupled receptors and other signaling pathways. This review summarizes the current data on the control of RGS function through regulated expression, intracellular localization, and covalent modification of RGS proteins, as related to cell function and the pathogenesis of diseases.

Potent and selective inhibition of angiotensin AT1 receptor signaling by RGS2: roles of its N-terminal domain

Emerging evidence indicates that R4/B subfamily RGS (regulator of G protein signaling) proteins play roles in functional regulation in the cardiovascular system. In this study, we compared effects of three R4/B subfamily proteins, RGS2, RGS4 and RGS5 on angiotensin AT1 receptor signaling, and investigated roles of the N-terminus of RGS2. In HEK293T cells expressing AT1 receptor stably, intracellular Ca(2+) responses induced by angiotensin II were much more strongly attenuated by RGS2 than by RGS4 and RGS5. N-terminally deleted RGS2 proteins lost this potent inhibitory effect. Replacement of the N-terminal residues 1-71 of RGS2 with the corresponding residues (1-51) of RGS5 decreased significantly the inhibitory effect. On the other hand, replacement of the residues 1-51 of RGS5 with the residues 1-71 of RGS2 increased the inhibitory effect dramatically. Furthermore, we investigated functional contribution of N-terminal subdomains of RGS2, namely, an N-terminal region (residues 16-55) with an amphipathic α helix domain (the subdomain N1), a probable non-specific membrane-targeting subdomain, and another region (residues 56-71) between the α helix and the RGS box (the subdomain N2), a probable GPCR-recognizing subdomain. RGS2 chimera proteins with the residues 1-33 or 34-52 of RGS5 showed weak inhibitory activity, and either of RGS5 chimera proteins with residues 1-55 or 56-71 of RGS2 showed strong inhibitory effects on AT1 receptor signaling. The present study indicates the essential roles of both N-terminal subdomains for the potent inhibitory activity of RGS2 on AT1 receptor signaling.

Sphingosine-1-phosphate regulates RGS2 and RGS16 mRNA expression in vascular smooth muscle cells

Regulator of G protein signalling (RGS) protein expression is altered under growth promoting conditions in vascular smooth muscle cells (VSMCs). Since sphingosine-1-phosphate (S1P) is an important growth stimulatory factor, we investigated whether stimulation of VSMCs with S1P results in alterations in mRNA expression levels of several RGS proteins and which signalling components are involved. VSMCs were stimulated with S1P and mRNA expression levels of RGS2, RGS3, RGS4, RGS5 and RGS16 were measured by real-time polymerase chain reaction. S1P caused a time-dependent up-regulation of RGS2 and RGS16 mRNA expression. FTY720-P, a S1P(1)/S1P(3-5) agonist, did not regulate RGS2 mRNA levels although it did up-regulate RGS16 mRNA expression. Pertussis toxin treatment revealed that the S1P-induced RGS16 expression was G(i/o)-dependent whereas up-regulation of RGS2 mRNA was not. Phosphatidylinositol 3-kinase, protein kinase C and mitogen-activated protein kinase kinase apparently were not involved in the S1P-induced up-regulation of both RGS proteins. The present study demonstrates that S1P induces RGS2 and RGS16 mRNA expression but uses distinct S1P receptor subtypes and signalling pathways to regulate expression of these RGS proteins.

A role for sphingosine kinase 1 in dextran sulfate sodium-induced colitis

The bioactive lipid sphingosine-1-phosphate (S1P) is emerging as an important mediator of immune and inflammatory responses. S1P formation is catalyzed by sphingosine kinase (SK), of which the SK1 isoenzyme is activated by tumor necrosis alpha (TNF-alpha). SK1 has been shown to be required for mediating TNF-alpha inflammatory responses in cells, including induction of cyclooxygenase 2 (COX-2). Because TNF-alpha and COX-2 are increased in patients with inflammatory bowel disease (IBD), we investigated the role of SK1 in a murine model of colitis. SK1(-/-) mice treated with dextran sulfate sodium (DSS) had significantly less blood loss, weight loss, colon shortening, colon histological damage, and splenomegaly than did wild-type (WT) mice. In addition, SK1(-/-) mice had no systemic inflammatory response. Moreover, WT but not SK1(-/-) mice treated with dextran sulfate sodium had significant increases in blood S1P levels, colon SK1 message and activity, and colon neutrophilic infiltrate. Unlike WT mice, SK1(-/-) mice failed to show colonic COX-2 induction despite an exaggerated TNF-alpha response; thus implicating for the first time SK1 in TNF-alpha-mediated COX-2 induction in vivo. Inhibition of SK1 may prove to be a valuable therapeutic target by inhibiting systemic and local inflammation in IBD.