Spinster 2, a sphingosine-1-phosphate transporter, plays a critical role in inflammatory and autoimmune diseases

Sphingosine 1-phosphate (S1P) is a pleiotropic bioactive sphingolipid metabolite that regulates numerous processes important for immune responses. S1P is made within cells and must be transported out of cells to exert its effects through activation of 5 specific cell surface GPCRs in an autocrine or paracrine fashion. Spinster 2 (Spns2) transports S1P out of cells, and its deletion in mice reduces circulating levels of S1P, alters immune cell trafficking, and induces lymphopenia. Here we examined the effects of Spns2 deletion on adaptive immune responses and in autoimmune disease models. Airway inflammation and hypersensitivity as well as delayed-type contact hypersensitivity were attenuated in Spns2(-/-) mice. Similarly, Spns2 deletion reduced dextran sodium sulfate- and oxazolone-induced colitis. Intriguingly, Spns2(-/-) mice were protected from the development of experimental autoimmune encephalopathy, a model of the autoimmune disease multiple sclerosis. Deletion of Spns2 also strongly alleviated disease development in collagen-induced arthritis. These results point to a broad role for Spns2-mediated S1P transport in the initiation and development of adaptive immune related disorders.

Inhibition of sphingosine 1-phosphate lyase for the treatment of rheumatoid arthritis: discovery of (E)-1-(4-((1R,2S,3R)-1,2,3,4-tetrahydroxybutyl)-1H-imidazol-2-yl)ethanone oxime (LX2931) and (1R,2S,3R)-1-(2-(isoxazol-3-yl)-1H-imidazol-4-yl)butane-1,2,3,4-tetraol (LX2932)

Sphingosine 1-phosphate lyase (S1PL) has been characterized as a novel target for the treatment of autoimmune disorders using genetic and pharmacological methods. Medicinal chemistry efforts targeting S1PL by direct in vivo evaluation of synthetic analogues of 2-acetyl-4(5)-(1(R),2(S),3(R),4-tetrahydroxybutyl)-imidazole (THI, 1) led to the discovery of 2 (LX2931) and 4 (LX2932). The immunological phenotypes observed in S1PL deficient mice were recapitulated by oral administration of 2 or 4. Oral dosing of 2 or 4 yielded a dose-dependent decrease in circulating lymphocyte numbers in multiple species and showed a therapeutic effect in rodent models of rheumatoid arthritis (RA). Phase I clinical trials indicated that 2, the first clinically studied inhibitor of S1PL, produced a dose-dependent and reversible reduction of circulating lymphocytes and was well tolerated at dose levels of up to 180 mg daily. Phase II evaluation of 2 in patients with active rheumatoid arthritis is currently underway.

Inhibition of sphingosine-1-phosphate lyase for the treatment of autoimmune disorders

During nearly a decade of research dedicated to the study of sphingosine signaling pathways, we identified sphingosine-1-phosphate lyase (S1PL) as a drug target for the treatment of autoimmune disorders. S1PL catalyzes the irreversible decomposition of sphingosine-1-phosphate (S1P) by a retro-aldol fragmentation that yields hexadecanaldehyde and phosphoethanolamine. Genetic models demonstrated that mice expressing reduced S1PL activity had decreased numbers of circulating lymphocytes due to altered lymphocyte trafficking, which prevented disease development in multiple models of autoimmune disease. Mechanistic studies of lymphoid tissue following oral administration of 2-acetyl-4(5)-(1(R),2(S),3(R),4-tetrahydroxybutyl)-imidazole (THI) 3 showed a clear relationship between reduced lyase activity, elevated S1P levels, and lower levels of circulating lymphocytes. Our internal medicinal chemistry efforts discovered potent analogues of 3 bearing heterocycles as chemical equivalents of the pendant carbonyl present in the parent structure. Reduction of S1PL activity by oral administration of these analogues recapitulated the phenotype of mice with genetically reduced S1PL expression.

Incomplete inhibition of sphingosine 1-phosphate lyase modulates immune system function yet prevents early lethality and non-lymphoid lesions

Background

S1PL is an aldehyde-lyase that irreversibly cleaves sphingosine 1-phosphate (S1P) in the terminal step of sphingolipid catabolism. Because S1P modulates a wide range of physiological processes, its concentration must be tightly regulated within both intracellular and extracellular environments.

Methodology

In order to better understand the function of S1PL in this regulatory pathway, we assessed the in vivo effects of different levels of S1PL activity using knockout (KO) and humanized mouse models.

Principal Findings

Our analysis showed that all S1PL-deficient genetic models in this study displayed lymphopenia, with sequestration of mature T cells in the thymus and lymph nodes. In addition to the lymphoid phenotypes, S1PL KO mice (S1PL^−/−) also developed myeloid cell hyperplasia and significant lesions in the lung, heart, urinary tract, and bone, and had a markedly reduced life span. The humanized knock-in mice harboring one allele (S1PL^H/−) or two alleles (S1PL^H/H) of human S1PL expressed less than 10 and 20% of normal S1PL activity, respectively. This partial restoration of S1PL activity was sufficient to fully protect both humanized mouse lines from the lethal non-lymphoid lesions that developed in S1PL^−/− mice, but failed to restore normal T-cell development and trafficking. Detailed analysis of T-cell compartments indicated that complete absence of S1PL affected both maturation/development and egress of mature T cells from the thymus, whereas low level S1PL activity affected T-cell egress more than differentiation.

Significance

These findings demonstrate that lymphocyte trafficking is particularly sensitive to variations in S1PL activity and suggest that there is a window in which partial inhibition of S1PL could produce therapeutic levels of immunosuppression without causing clinically significant S1P-related lesions in non-lymphoid target organs.

High-throughput screening of mouse knockout lines identifies true lean and obese phenotypes

We developed a high-throughput approach to knockout (KO) and phenotype mouse orthologs of the 5,000 potential drug targets in the human genome. As part of the phenotypic screen, dual-energy X-ray absorptiometry (DXA) technology estimates body-fat stores in eight KO and four wild-type (WT) littermate chow-fed mice from each line. Normalized % body fat (nBF) (mean KO % body fat/mean WT littermate % body fat) values from the first 2322 lines with viable KO mice at 14 weeks of age showed a normal distribution. We chose to determine how well this screen identifies body-fat phenotypes by selecting 13 of these 2322 KO lines to serve as benchmarks based on their published lean or obese phenotype on a chow diet. The nBF values for the eight benchmark KO lines with a lean phenotype were > or =1 s.d. below the mean for seven (perilipin, SCD1, CB1, MCH1R, PTP1B, GPAT1, PIP5K2B) but close to the mean for NPY Y4R. The nBF values for the five benchmark KO lines with an obese phenotype were >2 s.d. above the mean for four (MC4R, MC3R, BRS3, translin) but close to the mean for 5HT2cR. This screen also identifies novel body-fat phenotypes as exemplified by the obese kinase suppressor of ras 2 (KSR2) KO mice. These body-fat phenotypes were confirmed upon studying additional cohorts of mice for KSR2 and all 13 benchmark KO lines. This simple and cost-effective screen appears capable of identifying genes with a role in regulating mammalian body fat.

Cloning and characterization of Sel-1l, a murine homolog of the C. elegans sel-1 gene

The Notch signaling pathway regulates specification and proliferation in a variety of cell lineages in invertebrates and vertebrates. We have cloned a murine homolog of SEL-1, a key negative regulator of the Notch pathway in Caenorhabditis elegans. Murine SEL-1L (mSEL-1L) protein exhibits a high degree of similarity to SEL-1, including a signal peptide and the C-terminal region required for SEL-1 function in C. elegans. This mammalian homolog of sel-1 is widely expressed in adult mouse and human tissues, with particularly high levels in the pancreas. RNA in situ analysis of developing mouse embryos indicates that mSEL-1L is moderately expressed throughout the neural tube and dorsal root ganglia, with particularly high levels in the floor plate of the neural tube beginning at E10.5 and increasing at E11.5. Expression is high at E14.5 and E17.5 in the acini of the pancreas, and moderate in the epithelial cells of the gut villi. We localized the SEL-1L protein to the cytosol, possibly in intracellular vesicles, in a beta-islet-derived tumor cell line (RinM).

Isolation and identification of genes activating UAS2-dependent ADH2 expression in Saccharomyces cerevisiae

Two cis-acting elements have been identified that act synergistically to regulate expression of the glucose-repressed alcohol dehydrogenase 2 (ADH2) gene. UAS1 is bound by the trans-activator Adr1p. UAS2 is thought to be the binding site for an unidentified regulatory protein. A genetic selection based on a UAS2-dependent ADH2 reporter was devised to isolate genes capable of activating UAS2-dependent transcription. One set of UAS2-dependent genes contained SPT6/CRE2/SSN20. Multicopy SPT6 caused improper expression of chromosomal ADH2. A second set of UAS2-dependent clones contained a previously uncharacterized open reading frame designated MEU1 (Multicopy Enhancer of UAS2). A frame shift mutation in MEU1 abolished its ability to activate UAS2-dependent gene expression. Multicopy MEU1 expression suppressed the constitutive ADH2 expression caused by cre2-1. Disruption of MEU1 reduced endogenous ADH2 expression about twofold but had no effect on cell viability or growth. No homologues of MEU1 were identified by low-stringency Southern hybridization of yeast genomic DNA, and no significant homologues were found in the sequence data bases. A MEU1/beta-gal fusion protein was not localized to a particular region of the cell. MEU1 is linked to PPR1 on chromosome XII.

Synergistic activation of ADH2 expression is sensitive to upstream activation sequence 2 (UAS2) orientation, copy number and UAS1-UAS2 helical phasing

The alcohol dehydrogenase 2 (ADH2) gene of Saccharomyces cerevisiae is under stringent glucose repression. Two cis-acting upstream activation sequences (UAS) that function synergistically in the derepression of ADH2 gene expression have been identified. UAS1 is the binding site for the transcriptional regulator Adr1p. UAS2 has been shown to be important for ADH2 expression and confers glucose-regulated, ADR1-independent activity to a heterologous reporter gene. An analysis of point mutations within UAS2, in the context of the entire ADH2 upstream regulatory region, showed that the specific sequence of UAS2 is important for efficient derepression of ADH2, as would be expected if UAS2 were the binding site for a transcriptional regulatory protein. In the context of the ADH2 upstream regulatory region, including UAS1, working in concert with the ADH2 basal promoter elements, UAS2-dependent gene activation was dependent on orientation, copy number, and helix phase. Multimerization of UAS2, or its presence in reversed orientation, resulted in a decrease in ADH2 expression. In contrast, UAS2-dependent expression of a reporter gene containing the ADH2 basal promoter and coding sequence was enhanced by multimerization of UAS2 and was independent of UAS2 orientation. The reduced expression caused by multimerization of UAS2 in the native promoter was observed only in the presence of ADR1. Inhibition of UAS2-dependent gene expression by Adr1p was also observed with a UAS2-dependent ADH2 reporter gene. This inhibition increased with ADR1 copy number and required the DNA-binding activity of Adr1p. Specific but low-affinity binding of Adr1p to UAS2 in vitro was demonstrated, suggesting that the inhibition of UAS2-dependent gene expression observed in vivo could be a direct effect due to Adr1p binding to UAS2.

Adjacent upstream activation sequence elements synergistically regulate transcription of ADH2 in Saccharomyces cerevisiae

A 22-base-pair (bp) inverted repeat present in the ADH2 promoter is an upstream activation sequence (UAS1) which confers ADR1-dependent activation upon a heterologous Saccharomyces cerevisiae promoter. UAS1 was nonfunctional when placed within an intron 3' to the transcription start site. The 11-bp sequence which constitutes one-half of the UAS1 palindrome did not activate transcription in a single copy, as direct repeats, or in an inverted orientation opposite to that of ADH2 UAS1. Furthermore, two pairs of symmetrical point mutations within UAS1 significantly reduced activation. This result suggests that a specific orientation of sequences within UAS1 is necessary for ADR1-dependent activation. We determined that an ADR1-dependent complex was formed with UAS1 and, to a lesser extent, with the nonfunctional 11-bp half palindrome. However, the 11 bp did not confer UAS activity, suggesting that ADR1 binding is not sufficient for activation in vivo. ADR1 did not bind to mutant UAS1 sequences in vitro, indicating that their decreased activation is attributable to a reduced affinity of ADR1 for these sequences. We also identified an additional 20-bp ADH2 element (UAS2) that increased the expression of CYC1-lacZ 20-fold when combined with UAS1. UAS2 permitted ADR1-independent, glucose-regulated expression of the hybrid gene. Consistent with this observation, ADR1 did not form a detectable complex with UAS2. Deletion of UAS2 at the chromosomal ADH2 locus virtually abolished ADH2 derepression and had no effect on glucose repression.