Involvement of sphingosine-1-phosphate in glutamate secretion in hippocampal neurons

Sphingosine kinase 1/sphingosine-1-phosphate regulates the expression of interleukin-17A in activated microglia in cerebral ischemia/reperfusion

BACKGROUND

Microglial activation is one of the causative factors of neuroinflammation in cerebral ischemia/reperfusion (IR). Sphingosine kinase 1 (Sphk1), a key enzyme responsible for phosphorylating sphingosine into sphingosine-1-phosphate (S1P), plays an important role in the regulation of proinflammatory cytokines in activated microglia. Recent research demonstrated that S1P increased IL-17A-secretion and then worsened CNS (central nervous system) inflammation. Thus, in the present study, we sought to use microglial cells as the object of study to discuss the molecular mechanisms in Sphk1/S1P-regulated IL-17A-secretion in IR.

METHODS

We used immunofluorescence and confocal microscopy to detect whether Sphk1 is expressed in microglia after cerebral IR or oxygen-glucose deprivation (OGDR). Western blot analysis was used to estimate the total Sphk1 protein level at different time points after OGDR. To detect cytokine secretion in microglial supernatants in response to OGDR, we measured the concentration of IL-17A in the culture supernatants using an enzyme-linked immunosorbent assay (ELISA). To evaluate whether microglia subjected to OGDR exhibited neuronal injury, we used a commercially available terminal transferase-mediated deoxyuridine triphosphate-biotin nick end labeling (TUNEL) kit to detect apoptotic neurons.

RESULTS

Sphk1 was expressed in microglia in response to cerebral IR or OGDR at appointed time. Pre-injection with PF-543, an inhibitor of Sphk1, before IR clearly reduced the expression of Sphk1 in microglia relative to brain IR alone. The number of TUNEL-positive neurons was also decreased in the PF-543-pretreated animals before IR compared to the animals with IR alone. When S1P was administered in OGDR microglia, IL-17A expression and neuronal apoptosis were increased compared to OGDR alone and the administration of S1P alone. ELISA further confirmed the above results. Moreover, the inhibition of Sphk1 by siRNA reduced IL-17A production and relieved neuronal apoptosis in OGDR microglia.

CONCLUSION

These results indicated that Sphk1/S1P regulates the expression of IL-17A in activated microglia, inducing neuronal apoptosis in cerebral ischemia/reperfusion. The microglial Sphk1/S1P pathway may thus be a potential therapeutic target to control neuroinflammation in brain IR.

Regulation of neuronal communication by G protein-coupled receptors

Neuronal communication plays an essential role in the propagation of information in the brain and requires a precisely orchestrated connectivity between neurons. Synaptic transmission is the mechanism through which neurons communicate with each other. It is a strictly regulated process which involves membrane depolarization, the cellular exocytosis machinery, neurotransmitter release from synaptic vesicles into the synaptic cleft, and the interaction between ion channels, G protein-coupled receptors (GPCRs), and downstream effector molecules. The focus of this review is to explore the role of GPCRs and G protein-signaling in neurotransmission, to highlight the function of GPCRs, which are localized in both presynaptic and postsynaptic membrane terminals, in regulation of intrasynaptic and intersynaptic communication, and to discuss the involvement of astrocytic GPCRs in the regulation of neuronal communication.

Sphingosine 1-phosphate signaling pathway in inner ear biology. New therapeutic strategies for hearing loss?

Hearing loss is one of the most prevalent conditions around the world, in particular among people over 60 years old. Thus, an increase of this affection is predicted as result of the aging process in our population. In this context, it is important to further explore the function of molecular targets involved in the biology of inner ear sensory cells to better individuate new candidates for therapeutic application. One of the main causes of deafness resides into the premature death of hair cells and auditory neurons. In this regard, neurotrophins and growth factors such as insulin like growth factor are known to be beneficial by favoring the survival of these cells. An elevated number of published data in the last 20 years have individuated sphingolipids not only as structural components of biological membranes but also as critical regulators of key biological processes, including cell survival. Ceramide, formed by catabolism of sphingomyelin (SM) and other complex sphingolipids, is a strong inducer of apoptotic pathway, whereas sphingosine 1-phosphate (S1P), generated by cleavage of ceramide to sphingosine and phosphorylation catalyzed by two distinct sphingosine kinase (SK) enzymes, stimulates cell survival. Interestingly S1P, by acting as intracellular mediator or as ligand of a family of five distinct S1P receptors (S1P₁–S1P₅), is a very powerful bioactive sphingolipid, capable of triggering also other diverse cellular responses such as cell migration, proliferation and differentiation, and is critically involved in the development and homeostasis of several organs and tissues. Although new interesting data have become available, the information on S1P pathway and other sphingolipids in the biology of the inner ear is limited. Nonetheless, there are several lines of evidence implicating these signaling molecules during neurogenesis in other cell populations. In this review, we discuss the role of S1P during inner ear development, also as guidance for future studies.

Sphingosin-1-phosphate Receptor 1: a Potential Target to Inhibit Neuroinflammation and Restore the Sphingosin-1-phosphate Metabolism

BACKGROUND

Recent evidence suggests that an extreme shift may occur in sphingosine metabolism in neuroinflammatory contexts. Sphingosine 1-phosphate (S1P)-metabolizing enzymes (SMEs) regulate the level of S1P. We recently found that FTY720, a S1P analogue, and SEW2871, a selective S1P receptor 1 (S1P1) agonist, provide protection against neural damage and memory deficit in amyloid beta (Aβ)-injected animals. This study aimed to evaluate the effects of these two analogues on the expression of SMEs as well as their anti-inflammatory roles.

METHODS

Rats were treated with intracerebral lipopolysaccharide (LPS) or Aβ. Memory impairment was assessed by Morris water maze and the effects of drugs on SMEs as well as inflammatory markers, TNF- α and COX-II, were determined by immunoblotting.

RESULTS

Aβ and LPS differentially altered the expression profile of SMEs. In Aβ-injected animals, FTY720 and SEW2871 treatments exerted anti-inflammatory effects and restored the expression profile of SMEs, in parallel to our previous findings. In LPS animals however, in spite of anti-inflammatory effects of the two analogues, only FTY720 restored the levels of SMEs and prevented memory deficit.

CONCLUSION

The observed ameliorating effects of FTY720 and SEW7821 can be partly attributed to the interruption of the vicious cycle of abnormal S1P metabolism and neuro-inflammation. The close imitation of the FTY720 effects by SW2871 in Aβ-induced neuro-inflammation may highlight the attractive role of S1P1 as a potential target to restore S1P metabolism and inhibit inflammatory processes.

Emerging biology of sphingosine-1-phosphate: its role in pathogenesis and therapy

Membrane sphingolipids are metabolized to sphingosine-1-phosphate (S1P), a bioactive lipid mediator that regulates many processes in vertebrate development, physiology, and pathology. Once exported out of cells by cell-specific transporters, chaperone-bound S1P is spatially compartmentalized in the circulatory system. Extracellular S1P interacts with five GPCRs that are widely expressed and transduce intracellular signals to regulate cellular behavior, such as migration, adhesion, survival, and proliferation. While many organ systems are affected, S1P signaling is essential for vascular development, neurogenesis, and lymphocyte trafficking. Recently, a pharmacological S1P receptor antagonist has won approval to control autoimmune neuroinflammation in multiple sclerosis. The availability of pharmacological tools as well as mouse genetic models has revealed several physiological actions of S1P and begun to shed light on its pathological roles. The unique mode of signaling of this lysophospholipid mediator is providing novel opportunities for therapeutic intervention, with possibilities to target not only GPCRs but also transporters, metabolic enzymes, and chaperones.

Harnessing the power of yeast to elucidate the role of sphingolipids in metabolic and signaling processes pertinent to psychiatric disorders

The development of therapies for neuropsychiatric disorders is hampered by the lack of understanding of the mechanisms underlying their pathologies. While aberrant sphingolipid metabolism is associated with psychiatric illness, the role of sphingolipids in these disorders is not understood. The genetically tractable yeast model can be exploited in order to elucidate the cellular consequences of sphingolipid perturbation. Hypotheses generated from studies in yeast and tested in mammalian cells may contribute to our understanding of the role of sphingolipids in psychiatric disorders and to the development of new treatments. Here, we compare sphingolipid metabolism in yeast and mammalian cells, discuss studies implicating sphingolipids in psychiatric disorders and propose approaches that utilize yeast in order to elucidate sphingolipid function and identify drugs that target sphingolipid synthesis.

Ongoing activation of sphingosine 1-phosphate receptors mediates maturation of exosomal multivesicular endosomes

During late endosome maturation, cargo molecules are sorted into intralumenal vesicles (ILVs) of multivesicular endosomes (MVEs), and are either delivered to lysosomes for degradation or fused with the plasma membranes for exosome release. The mechanism underlying formation of exosomal ILVs and cargo sorting into ILVs destined for exosome release is still unclear. Here we show that inhibitory G protein (Gi)-coupled sphingosine 1-phosphate (S1P) receptors regulate exosomal MVE maturation. Gi-coupled S1P receptors on MVEs are constitutively activated through a constant supply of S1P via autocrine activation within organelles. We also found that the continuous activation of Gi-coupled S1P receptors on MVEs is essential for cargo sorting into ILVs destined for exosome release. Our results reveal a mechanism underlying ESCRT-independent maturation of exosomal MVEs.

Coupling between endocytosis and sphingosine kinase 1 recruitment

Genetic studies have suggested a functional link between cholesterol/sphingolipid metabolism and endocytic membrane traffic. Here we show that perturbing the cholesterol/sphingomyelin balance in the plasma membrane results in the massive formation of clusters of narrow endocytic tubular invaginations positive for N-BAR proteins. These tubules are intensely positive for sphingosine kinase 1 (SPHK1). SPHK1 is also targeted to physiologically occurring early endocytic intermediates, and is highly enriched in nerve terminals, which are cellular compartments specialized for exo/endocytosis. Membrane recruitment of SPHK1 involves a direct, curvature-sensitive interaction with the lipid bilayer mediated by a hydrophobic patch on the enzyme's surface. The knockdown of SPHKs results in endocytic recycling defects, and a mutation that disrupts the hydrophobic patch of Caenorhabditis elegans SPHK fails to rescue the neurotransmission defects in loss-of-function mutants of this enzyme. Our studies support a role for sphingosine phosphorylation in endocytic membrane trafficking beyond the established function of sphingosine-1-phosphate in intercellular signalling.

Oral fingolimod reduces glutamate-mediated intracortical excitability in relapsing-remitting multiple sclerosis

OBJECTIVE

Fingolimod is an effective disease modifying therapy for multiple sclerosis (MS). Beyond its main action on peripheral lymphocytes, several noteworthy side effects have been demonstrated in vitro, among which modulation of neural excitability. Our aim was to explore cortical excitability in vivo in patients treated with fingolimod 0.5mg/day.

METHODS

Paired-pulse TMS was applied on the left primary motor cortex in 13 patients affected by relapsing-remitting MS, the day before the first dose of fingolimod (T0) and 60days later (T1). Resting motor threshold, baseline motor evoked potentials, short interval intracortical inhibition (at 1, 3, 5ms) and intracortical facilitation (at 7, 9, 11 and 13ms) were estimated at T0 and T1.

RESULTS

Intracortical facilitation was reduced at T1, without any changes in short interval intracortical inhibition.

CONCLUSIONS

Fingolimod selectively reduced intracortical facilitation, which is mainly mediated by glutamate.

SIGNIFICANCE

This is the first in vivo confirmation of the effects of fingolimod on glutamatergic drive in treated humans. Our results suggest a novel neuromodulatory activity of fingolimod with potential effect on glutamate-mediated excitotoxicity in vivo, as already seen in animal models.

Temporal expression of transporters and receptors in a rat primary co-culture blood-brain barrier model

1. The more relevant primary co-cultures of brain microvessel endothelial cells and astrocytes (BMEC) are less utilized for screening of potential CNS uptake when compared to intestinal and renal cell lines. 2. In this study, we characterized the temporal mRNA expression of major CNS transporters and receptors, including the transporter regulators Pxr, Ahr and Car in a rat BMEC co-cultured model. Permeability was compared with the Madin-Darby canine kidney (MDCKII)-MDR1 cell line and rat brain in situ perfusion model. 3. Our data demonstrated differential changes in expression of individual transporters and receptors over the culture period. Expression of ATP-binding cassette transporters was better retained than that of solute carrier transporters. The insulin receptor (IR) was best maintained among investigated receptors. AhR demonstrated high mRNA expression in rat brain capillaries and expression was better retained than Pxr or Car in culture. Mdr1b expression was up-regulated during primary culture, albeit Mdr1a mRNA levels were much higher. P-gp and Bcrp-1 were highly expressed and functional in this in vitro system. 4. Permeability measurements with 18 CNS marketed drugs demonstrated weak correlation between rBMEC model and rat in situ permeability and moderate correlation with MDCKII-MDR1 cells. 5. We have provided appropriate methodologies, as well as detailed and quantitative characterization data to facilitate improved understanding and rational use of this in vitro rat BBB model.

Loss of the neuroprotective factor Sphingosine 1-phosphate early in Alzheimer's disease pathogenesis

Background

The greatest genetic risk factor for late-onset Alzheimer's disease (AD) is the ϵ4 allele of Apolipoprotein E (ApoE). ApoE regulates secretion of the potent neuroprotective signaling lipid Sphingosine 1-phosphate (S1P). S1P is derived by phosphorylation of sphingosine, catalysed by sphingosine kinases 1 and 2 (SphK1 and 2), and SphK1 positively regulates glutamate secretion and synaptic strength in hippocampal neurons. S1P and its receptor family have been subject to intense pharmacological interest in recent years, following approval of the immunomodulatory drug Fingolimod, an S1P mimetic, for relapsing multiple sclerosis.

Results

We quantified S1P levels in six brain regions that are differentially affected by AD pathology, in a cohort of 34 post-mortem brains, divided into four groups based on Braak neurofibrillary tangle staging. S1P declined with increasing Braak stage, and this was most pronounced in brain regions most heavily affected by AD pathology. The S1P/sphingosine ratio was 66% and 64% lower in Braak stage III/IV hippocampus (p = 0.010) and inferior temporal cortex (p = 0.014), respectively, compared to controls. In accordance with this change, both SphK1 and SphK2 activity declined with increasing Braak pathology in the hippocampus (p = 0.032 and 0.047, respectively). S1P/sphingosine ratio was 2.5-fold higher in hippocampus of ApoE2 carriers compared to ApoE4 carriers, and multivariate regression showed a significant association between APOE genotype and hippocampal S1P/sphingosine (p = 0.0495), suggesting a new link between APOE genotype and pre-disposition to AD.

Conclusions

This study demonstrates loss of S1P and sphingosine kinase activity early in AD pathogenesis, and prior to AD diagnosis. Our findings establish a rationale for further exploring S1P receptor pharmacology in the context of AD therapy.

Sphingosine-1 phosphate and central nervous system

The development of fingolimod, an unselective functional antagonist of the interactions between sphingosine 1 phosphate (S1P) and sphingosine 1 phosphate receptors (S1PRs), as the first oral therapy for multiple sclerosis (MS) has been a milestone. The parallel intensive research on the role of S1P, sphingosine kinases, and the five known S1PRs, their tissue distribution and expression in physiological and pathological conditions have led to a wide range of interesting findings. The initial focus of this research in the context of developing fingolimod as a treatment of MS has been on its immunological effects. The wide distribution and important roles of sphingosine, its metabolites, and their receptors in the central nervous system (CNS) in general, in myelin, and in all cell types of this organ have spurred interest to examine S1P and its five receptors in the brain as well. The present review will concentrate on the latter area and give a brief overview of what is known about S1P/S1PR interactions in the CNS in physiological and pathological conditions.

Insulin protects apoptotic cardiomyocytes from hypoxia/reoxygenation injury through the sphingosine kinase/sphingosine 1-phosphate axis

OBJECTIVE

Experimental and clinical studies have shown that administration of insulin during reperfusion is cardioprotective, but the mechanisms underlying this effect are still unknown. In this study, the ability of insulin to protect apoptotic cardiomyocytes from hypoxia/reoxygenation injury using the sphingosine kinase/sphingosine 1-phosphate axis was investigated.

METHODS AND RESULTS

Rat cardiomyocytes were isolated and subjected to hypoxia and reoxygenation. [γ-32P] ATP was used to assess sphingosine kinase activity. Insulin was found to increase sphingosine kinase activity. Immunocytochemistry and Western blot analysis showed changes in the subcellular location of sphingosine kinase 1 from cytosol to the membrane in cardiomyocytes. Insulin caused cardiomyocytes to accumulate of S1P in a dose-dependent manner. FRET efficiency showed that insulin also transactivates the S1P1 receptor. TUNEL staining showed that administration of insulin during reoxygenation could to reduce the rate of reoxygenation-induced apoptosis, which is a requirement for SphK 1 activity. It also reduced the rate of activation of the S1P receptor and inhibited hypoxia/reoxygenation-induced cell death in cardiomyocytes.

CONCLUSION

The sphingosine kinase 1/sphingosine 1-phosphate/S1P receptor axis is one pathway through which insulin protects rat cardiomyocytes from apoptosis induced by hypoxia/reoxygenation injury.

Glutamate mediates the function of melanocortin receptor 4 on Sim1 neurons in body weight regulation

The melanocortin receptor 4 (MC4R) is a well-established mediator of body weight homeostasis. However, the neurotransmitter(s) that mediate MC4R function remain largely unknown; as a result, little is known about the second-order neurons of the MC4R neural pathway. Single-minded 1 (Sim1)-expressing brain regions, which include the paraventricular nucleus of hypothalamus (PVH), represent key brain sites that mediate melanocortin action. We conditionally restored MC4R expression in Sim1 neurons in the background of Mc4r-null mice. The restoration dramatically reduced obesity in Mc4r-null mice. The anti-obesity effect was completely reversed by selective disruption of glutamate release from those same Sim1 neurons. The reversal was caused by lower energy expenditure and hyperphagia. Corroboratively, selective disruption of glutamate release from adult PVH neurons led to rapid obesity development via reduced energy expenditure and hyperphagia. Thus, this study establishes glutamate as the primary neurotransmitter that mediates MC4Rs on Sim1 neurons in body weight regulation.

Therapeutic potential of targeting SK1 in human cancers

Sphingosine kinase 1 (SK1) is a lipid enzyme with oncogenic properties that converts the proapoptotic lipids ceramide and sphingosine into the antiapoptotic lipid sphingosine-1-phosphate and activates the signal transduction pathways that lead to cell proliferation, migration, the activation of the inflammatory response, and the impairment of apoptosis. There is compelling evidence that SK1 activation contributes to cancer progression leading to increased oncogenic transformation, tumor growth, resistance to therapies, tumor neovascularization, and metastatic spread. High levels of SK1 expression or activity have been associated with a poor prognosis in several human cancers. Recent studies using cancer cell and mouse models demonstrate a significant potential for SK1-targeting therapies to synergize with the effects of chemotherapy and radiotherapy; however, until recently the absence of clinically applicable SK1 inhibitors has limited the translation of these findings into patients. With the recent discovery of SK1 inhibiting properties of a clinically approved drug FTY720 (Fingolimod), SK1 has gained significant attention from both clinicians and the pharmaceutical industry and it is hoped that trials of newly developed SK1 inhibitors may follow soon. This review provides an overview of the SK1 signaling, its relevance to cancer progression, and the potential clinical significance of targeting SK1 for improved local or systemic control of human cancers.

Activation of sphingosine 1-phosphate receptor-1 by SEW2871 improves cognitive function in Alzheimer's disease model rats

Sphingosine-1 phosphate (S1P) is involved in a variety of cellular processes via activation of S1P receptors (S1PRs; S1PR1 to S1PR5) that are highly expressed in the brain. It has been shown that the level of S1P is reduced in the brain of Alzheimer's disease (AD) patients. However, there is no study designed to evaluate the expression of S1PRs in AD brains. The objectives of the present work are (1) to examine the expression of S1PR1-3 in the hippocampus of beta amyloid (Aβ) 1-42 injected rats and (2) to clarify the effects of chronic S1PR1 activation on S1PR1-3 levels, spatial memory deficit and hippocampal damage in AD rats. SEW2871, the S1PR1 selective agonist, repeatedly was injected intraperitoneally during a period of two weeks. Upon Western Blot data bilateral intrahippocampal injection of Aβ1-42 decreased the expression of S1PR1 while increased S1PR2 level and did not affect that of S1PR3. We found that chronic administration of SEW2871 inhibited the reduction of S1PR1 expression and ameliorated spatial memory impairment in the Morris water maze task in rats. In addition, SEW2871 attenuated the Aβ1-42-induced hippocampal neuronal loss according to Nissl staining findings. Data in the current study highlights the importance of S1PR1 signaling pathway deregulation in AD development and suggests that activation of S1PR1 may represent a potential approach for developing new therapeutics to manage memory deficit and apoptosis associated with neurodegenerative disorders such as AD.

Localized sphingolipid signaling at presynaptic terminals is regulated by calcium influx and promotes recruitment of priming factors

Activity-dependent changes in presynaptic function represent a critical mechanism by which synaptic strength is controlled. However, how changes in synaptic activity couple to presynaptic components to control synaptic vesicle release and recycling are poorly understood. Sphingosine kinase (SphK) is a sphingolipid metabolic enzyme whose activity-dependent recruitment to membrane regions within presynaptic terminals promotes neurotransmitter release. Here, we show that synaptic recruitment of SPHK-1, the SphK ortholog in Caenorhabditis elegans, is mediated by presynaptic calcium influx. Quantitative fluorescence imaging of live presynaptic terminals reveals that blocking presynaptic calcium influx reduces synaptic SPHK-1 abundance whereas increasing calcium influx increases SPHK-1 synaptic abundance. CALM-1, the calcium and integrin binding protein ortholog, colocalizes with SPHK-1 at release sites and regulates muscarinic-mediated synaptic SPHK-1 recruitment. We identify two additional sphingolipid metabolic enzymes that are concentrated at presynaptic terminals, and mutants lacking one of these, HYL-1/ceramide synthase, have defects in synaptic transmission and in synaptic vesicle cycling. Finally, we show that SPHK-1 activity is required for the recruitment of the priming protein UNC-13/Munc13 to presynaptic terminals following activation by muscarinic signaling. These findings suggest that calcium-dependent regulation of local S1P metabolism at synapses may be an important mechanism by which synaptic vesicle priming factors are recruited to release sites to promote synaptic transmission.

Fingolimod: direct CNS effects of sphingosine 1-phosphate (S1P) receptor modulation and implications in multiple sclerosis therapy

Fingolimod is the first oral disease-modifying therapy approved for relapsing forms of multiple sclerosis (MS). Following phosphorylation in vivo, the active agent, fingolimod phosphate (fingolimod-P), acts as a sphingosine 1-phosphate (S1P) receptor modulator, binding with high affinity to four of the five known S1P receptors (S1P1, S1P3, S1P4 and S1P5). The mechanism of action of fingolimod in MS has primarily been considered as immunomodulatory, whereby fingolimod-P modulates S1P1 on lymphocytes, selectively retaining autoreactive lymphocytes in lymph nodes to reduce damaging infiltration into the central nervous system (CNS). However, emerging evidence indicates that fingolimod has direct effects in the CNS in MS. For example, in the MS animal model of experimental autoimmune encephalomyelitis (EAE), fingolimod is highly efficacious in both a prophylactic and therapeutic setting, yet becomes ineffective in animals selectively deficient for S1P1 on astrocytes, despite maintained normal immunologic receptor expression and functions, and S1P-mediated immune activities. Here we review S1P signaling effects relevant to MS in neural cell types expressing S1P receptors, including astrocytes, oligodendrocytes, neurons, microglia and dendritic cells. The direct effects of fingolimod on these CNS cells observed in preclinical studies are discussed in view of the functional consequences of reducing neurodegenerative processes and promoting myelin preservation and repair. The therapeutic implications of S1P modulation in the CNS are considered in terms of the clinical outcomes of MS, such as reducing MS-related brain atrophy, and other CNS disorders. Additionally, we briefly outline other existing and investigational MS therapies that may also have effects in the CNS.

FTY720 (fingolimod) attenuates beta-amyloid peptide (Aβ42)-induced impairment of spatial learning and memory in rats

Imbalanced lipid metabolism and increase in the ceramide-to-S1P ratio in the brain have been postulated to play a role in amyloidogenesis, neuroinflammatory reactions, and neuronal apoptosis in Alzheimer's disease (AD) pathology. FTY720, the immunomodulatory sphingosine 1-phosphate (S1P) analog, has recently gained interest because of its CNS-directed effects. In addition to its immunomodulatory functions in multiple sclerosis, FTY720 possesses anti-inflammatory and neuroprotective roles in different cerebral ischemia models. In the present study, we examined the effects of FTY720 in a rat model of AD. Memory deficit was induced by bilateral intrahippocampus injection of beta-amyloid peptide (Aβ(42)) and examined through the Morris water maze test. The extent of histological injury in the hippocampus and the activation of caspase-3 were determined respectively by Nissl staining and Western blotting. Chronic daily administration of FTY720 (1 mg/kg, i.p., 14 days) significantly attenuated the Aβ(42)-induced learning and memory impairment and prevented the hippocampus neuronal damage as well as caspase-3 activation. These data show for the first time that FTY720 has a beneficial effect in restoring memory loss in Aβ(42)-induced neurotoxicity and also suggest that S1P receptors and signaling pathways may provide a potential target for the treatment of AD.

Optimized protocols to analyze sphingosine-1-phosphate signal transduction pathways during acrosomal exocytosis in human sperm

Regulated secretion is a central issue for the specific function of many cells; for instance, mammalian sperm acrosomal exocytosis is essential for egg fertilization. Sphingosine 1-phosphate is a bioactive sphingolipid that regulates crucial physiological processes. We have recently reported that sphingosine 1-phosphate and sphingosine kinase are involved in a novel signaling pathway leading to acrosomal exocytosis (Suhaiman L et al., J Biol Chem 285:1630-16314, 2010). Acrosomal exocytosis in mammalian sperm is a regulated secretion with unusual characteristics. We therefore employed biochemical functional assays to assess the sphingolipid signaling in both permeabilized and nonpermeabilized sperm. The exocytosis of the acrosomal content is regulated by Ca(2+). During exocytosis, changes in [Ca(2+)]i occur induced by either Ca(2+)-influx or Ca(2+)-mobilization from intracellular stores. By using single cell [Ca(2+)] measurements, we detected intracellular Ca(2+) changes after sphingosine 1-phosphate treatment. Additionally, measuring sphingosine kinase activity, we determined that sphingosine 1-phosphate levels increase after an exocytotic stimulus.This chapter is designed to provide the user with sufficient background to analyze sphingosine 1--phosphate signal transduction pathways during acrosomal exocytosis in human sperm.

Fingolimod protects cultured cortical neurons against excitotoxic death

Fingolimod (FTY720), a novel drug approved for the treatment of relapsing-remitting multiple sclerosis, activates different sphingosine-1-phosphate receptor (S1PR) subtypes. Its primary mechanism of action is to reduce the egress of T lymphocytes from secondary lymphoid organs, thus restraining neuroinflammation and autoimmunity. However, recent evidence suggests that the action of FTY720 involves S1PRs expressed by cells resident in the CNS, including neurons. Here, we examined the effect of FTY720, its active metabolite, FTY720-P, and sphingosine-1-phosphate (S1P) on neuronal viability using a classical in vitro model of excitotoxic neuronal death. Mixed cultures of mouse cortical cells were challenged with toxic concentrations of N-methyl-d-aspartate (NMDA) for 10 min, and neuronal death was assessed 20 h later. FTY720, FTY720-P, and S1P were all neuroprotective when applied 18-20 h prior to the NMDA pulse. Neuroprotection was attenuated by pertussis toxin, and inhibited by the selective type-1 S1PR (S1P1R) antagonist, W146, and by inhibitors of the mitogen associated protein kinase (MAPK) and the phosphatidylinositol-3-kinase (PtdIns-3-K) pathways. Both FTY720 and FTY720-P retained their protective activity in pure cultures of mouse or rat cortical neurons. These data offer the first direct demonstration that FTY720 and its active metabolite protect neurons against excitotoxic death.

Expression of sphingosine 1-phosphate receptors in the rat dorsal root ganglia and defined single isolated sensory neurons

Previously, we demonstrated that sphingosine 1-phosphate (S1P) increased the excitability of small-diameter sensory neurons, in part, through activation of S1P receptor 1 (S1PR(1)), suggesting that other S1PRs can modulate neuronal excitability. Therefore, studies were undertaken to establish the expression profiles of S1PRs in the intact dorsal root ganglion (DRG) and in defined single isolated sensory neurons. To determine mRNA expression of S1PRs in the DRG, SYBR green quantitative PCR (qPCR) was used. To determine the expression of S1PR mRNAs in single neurons of defined diameters, a preamplification protocol utilizing Taqman primer and probes was used to enhance the sensitivity of detection. The preamplification protocol also permitted detection of mRNA for two hallmark neuronal receptor/ion channels, TRPV1 and P(2)X(3). Expression profiles of S1PR mRNA isolated from lung and brain were used as positive control tissues. In the intact DRG, the order of expression of S1PRs was S1PR(3)>>R(1)≈R(2)>R(5)≈R(4). In the single neurons, the expression of S1PRs was quite variable with some neurons expressing all five subtypes, whereas some expressing only one subtype. In contrast to the DRG, S1PR(1) was the highest expressing subtype in 10 of the 18 small-, medium-, and large-diameter sensory neurons. S1PR(1) was the second highest expressor in -50% of those remaining neurons. Overall, in the single neurons, the order of expression was S1PR(1)>>R(3)≈R(5)>R(4)>R(2). The results obtained from the single defined neurons are consistent with our previous findings wherein S1PR(1) plays a prominent but not exclusive role in the enhancement of neuronal excitability.

Vaccine Development to Treat Alzheimer's Disease Neuropathology in APP/PS1 Transgenic Mice

A novel vaccine addressing the major hallmarks of Alzheimer's disease (AD), senile plaque-like deposits of amyloid beta-protein (Aβ), neurofibrillary tangle-like structures, and glial proinflammatory cytokines, has been developed. The present vaccine takes a new approach to circumvent failures of previous ones tested in mice and humans, including the Elan-Wyeth vaccine (AN1792), which caused massive T-cell activation, resulting in a meningoencephalitis-like reaction. The EB101 vaccine consists of Aβ_1-42 delivered in a novel immunogen-adjuvant composed of liposomes-containing sphingosine-1-phosphate (S1P). EB101 was administered to APPswe/PS1dE9 transgenic mice before and after AD-like pathological symptoms were detectable. Treatment with EB101 results in a marked reduction of Aβ plaque burden, decrease of neurofibrillary tangle-like structure density, and attenuation of astrocytosis. In this transgenic mouse model, EB101 reduces the basal immunological interaction between the T cells and immune activation markers in the affected hippocampal/cortical areas, consistent with decreased amyloidosis-induced inflammation. Therefore, immunization with EB101 prevents and reverses AD-like neuropathology in a significant manner by halting disease progression without developing behavioral spatial deficits in transgenic mice.

Sphingosine-1-phosphate receptors as emerging targets for treatment of pain

Lysolipids are important mediators of cellular communication in multiple physiological processes. Sphingosine-1-phosphate (S1P) is a major lysolipid in many organs, including the central nervous system (CNS). This commentary discusses recent findings on the role of S1P in regulating pain perception, and highlights advances and challenges in the field. S1P interacts with multiple cellular targets, including G-protein-coupled receptors. Known S1P receptors include five types, four of which are expressed in the CNS (S1P(1,2,3,5)) where they are localized on neurons and glia. S1P receptor-mediated G-protein activation has been demonstrated throughout the CNS, including regions that regulate nociception. S1P receptors couple to multiple G-proteins to produce various intracellular responses, and can mediate both excitatory and inhibitory neuromodulation, depending on the receptor type and cellular context. Both antinociceptive and pro-nociceptive effects of S1P have been reported, and both actions can involve S1P(1) receptors. Current evidence suggests that antinociception is mediated by CNS neurons, whereas pro-nociception is mediated by primary afferent neurons or immune cells in the periphery, or CNS glia. Nonetheless, peripheral administration of the S1P(1,3,4,5) agonist pro-drug, FTY720, produces antinociception. FTY720 is approved to treat multiple sclerosis, and produces potent anti-inflammatory effects, which suggests potential utility for painful autoimmune diseases. Furthermore, evidence suggests that the S1P system interacts with other pain-modulatory systems, such as endogenous cannabinoid and opioid systems, and putative novel sphingolipid targets in the CNS. These findings suggest that drugs targeting the S1P system could be developed as novel analgesics, either as monotherapy or potential adjuncts to established analgesics.

Recruitment of sphingosine kinase to presynaptic terminals by a conserved muscarinic signaling pathway promotes neurotransmitter release

Sphingolipids are potent lipid second messengers that regulate cell differentiation, migration, survival, and secretion, and alterations in sphingolipid signaling have been implicated in a variety of diseases. However, how sphingolipid levels are regulated, particularly in the nervous system, remains poorly understood. Here, we show that the generation of sphingosine-1-phosphate by sphingosine kinase (SphK) promotes neurotransmitter release. Electrophysiological, imaging, and behavioral analyses of Caenorhabditis elegans mutants lacking sphingosine kinase sphk-1 indicate that neuronal development is normal, but there is a significant defect in neurotransmitter release from neuromuscular junctions. SPHK-1 localizes to discrete, nonvesicular regions within presynaptic terminals, and this localization is critical for synaptic function. Muscarinic agonists cause a rapid increase in presynaptic SPHK-1 abundance, whereas reduction of endogenous acetylcholine production results in a rapid decrease in presynaptic SPHK-1 abundance. Muscarinic regulation of presynaptic SPHK-1 abundance is mediated by a conserved presynaptic signaling pathway composed of the muscarinic acetylcholine receptor GAR-3, the heterotrimeric G protein Gαq, and its effector, Trio RhoGEF. SPHK-1 activity is required for the effects of muscarinic signaling on synaptic transmission. This study shows that SPHK-1 promotes neurotransmitter release in vivo and identifies a novel muscarinic pathway that regulates SphK abundance at presynaptic terminals.

Lysophospholipids and their receptors in the central nervous system

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), two of the best-studied lysophospholipids, are known to influence diverse biological events, including organismal development as well as function and pathogenesis within multiple organ systems. These functional roles are due to a family of at least 11 G protein-coupled receptors (GPCRs), named LPA(1-6) and S1P(1-5), which are widely distributed throughout the body and that activate multiple effector pathways initiated by a range of heterotrimeric G proteins including G(i/o), G(12/13), G(q) and G(s), with actual activation dependent on receptor subtypes. In the central nervous system (CNS), a major locus for these signaling pathways, LPA and S1P have been shown to influence myriad responses in neurons and glial cell types through their cognate receptors. These receptor-mediated activities can contribute to disease pathogenesis and have therapeutic relevance to human CNS disorders as demonstrated for multiple sclerosis (MS) and possibly others that include congenital hydrocephalus, ischemic stroke, neurotrauma, neuropsychiatric disorders, developmental disorders, seizures, hearing loss, and Sandhoff disease, based upon the experimental literature. In particular, FTY720 (fingolimod, Gilenya, Novartis Pharma, AG) that becomes an analog of S1P upon phosphorylation, was approved by the FDA in 2010 as a first oral treatment for MS, validating this class of receptors as medicinal targets. This review will provide an overview and update on the biological functions of LPA and S1P signaling in the CNS, with a focus on results from studies using genetic null mutants for LPA and S1P receptors. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

The Role of ATP-Binding Cassette Transporters in Neuro-Inflammation: Relevance for Bioactive Lipids

ATP-binding cassette (ABC) transporters are highly expressed by brain endothelial cells that form the blood–brain barrier (BBB). These efflux pumps play an important role in maintaining brain homeostasis as they actively hinder the entry of unwanted blood-derived compounds into the central nervous system (CNS). Consequently, their high activity at the BBB has been a major hurdle for the treatment of several brain diseases, as they prevent numerous drugs to reach their site of action within the brain. Importantly, recent data indicate that endogenous substrates for ABC transporters may include inflammatory mediators, such as prostaglandins, leukotrienes, cytokines, chemokines, and bioactive lipids, suggesting a potential role for ABC transporters in immunological responses, and more specifically in inflammatory brain disorders, such as multiple sclerosis (MS). In this review, we will give a comprehensive overview of recent findings that illustrate this novel role for ABC transporters in neuro-inflammatory processes. Moreover, we will provide first insights into underlying mechanisms and focus on the importance for bioactive lipids, in particular platelet-activating factor, herein. A thorough understanding of these events may form the basis for the development for selective treatment modalities to dampen the neuro-inflammatory attack in MS and thereby reducing tissue damage.

Sphingosine 1-phosphate, sphingosine kinase and autocrine calcium signalling in thyroid cells

In many cell types metabolites of sphingomyelin have a profound role in cellular signalling. One particular field where these derivatives have obtained a crucial role is calcium signalling. This is an interesting aspect on how lipids may wield their physiological role, as calcium is probably one of the most versatile signalling molecules in the cell, and modulation of calcium signalling may have profound effects on cellular physiology. In this review we discuss a novel aspect of sphingolipid signalling, i.e. the autocrine role of sphingosine 1-phosphate (S1P) in regulating calcium entry in thyroid cells. Although many investigations have highlighted the importance of S1P as a regulator of both calcium release from the endoplasmic reticulum and calcium entry through plasma membrane channels, the autocrine mechanism presented here introduces a new aspect of S1P signalling in thyroid cells. This mechanism may be physiologically relevant in many other cell types, including cancer cells.

Microvesicles released from microglia stimulate synaptic activity via enhanced sphingolipid metabolism

Microvesicles (MVs) released into the brain microenvironment are emerging as a novel way of cell-to-cell communication. We have recently shown that microglia, the immune cells of the brain, shed MVs upon activation but their possible role in microglia-to-neuron communication has never been explored. To investigate whether MVs affect neurotransmission, we analysed spontaneous release of glutamate in neurons exposed to MVs and found a dose-dependent increase in miniature excitatory postsynaptic current (mEPSC) frequency without changes in mEPSC amplitude. Paired-pulse recording analysis of evoked neurotransmission showed that MVs mainly act at the presynaptic site, by increasing release probability. In line with the enhancement of excitatory transmission in vitro, injection of MVs into the rat visual cortex caused an acute increase in the amplitude of field potentials evoked by visual stimuli. Stimulation of synaptic activity occurred via enhanced sphingolipid metabolism. Indeed, MVs promoted ceramide and sphingosine production in neurons, while the increase of excitatory transmission induced by MVs was prevented by pharmacological or genetic inhibition of sphingosine synthesis. These data identify microglia-derived MVs as a new mechanism by which microglia influence synaptic activity and highlight the involvement of neuronal sphingosine in this microglia-to-neuron signalling pathway.

Frequent spontaneous seizures followed by spatial working memory/anxiety deficits in mice lacking sphingosine 1-phosphate receptor 2

The diverse physiological effects of sphingosine 1-phosphate (S1P) are mostly mediated by its five cognate G protein-coupled receptors, S1P(1)-S1P(5), which have attracted much attention as future drug targets. To gain insight into S1P(2)-mediated signaling, we analyzed frequent spontaneous seizures in S1P(2)-deficient (S1P(2)(-/-)) mice obtained after several backcrosses onto a C57BL/6N background. Full-time video recording of 120 S1P(2)(-/-) mice identified 420 seizures both day and night between postnatal days 25 and 45, which were accompanied by high-voltage synchronized cortical discharges and a series of typical episodes: wild run, tonic-clonic convulsion, freezing, and, occasionally, death. Nearly 40% of 224 S1P(2)(-/-) mice died after such seizures, while the remaining 60% of the mice survived to adulthood; however, approximately half of the deliveries from S1P(2)(-/-) pregnant mice resulted in neonatal death. In situ hybridization revealed exclusive s1p(2) expression in the hippocampal pyramidal/granular neurons of wild-type mice, and immunohistochemistry/microarray analyses identified enhanced gliosis in the whole hippocampus and its neighboring neocortex in seizure-prone adult S1P(2)(-/-) mice. Seizure-prone adult S1P(2)(-/-) mice displayed impaired spatial working memory in the eight-arm radial maze test and increased anxiety in the elevated plus maze test, whereas their passive avoidance learning memory performance in the step-through test and hippocampal long-term potentiation was indistinguishable from that of wild-type mice. Our findings suggest that blockade of S1P(2) signaling may cause seizures/hippocampal insults and impair some specific central nervous system functions.

Therapeutic potential of targeting sphingosine kinase 1 in prostate cancer

Sphingosine kinase 1 (SK1) is a lipid enzyme with oncogenic properties that converts the proapoptotic lipid sphingosine into the antiapoptotic lipid sphingosine-1-phosphate, which activates the signal transduction pathways that lead to cell proliferation, migration, activation of the inflammatory response and impairment of apoptosis. Compelling evidence suggests that SK1 activation contributes to cancer progression leading to increased oncogenic transformation, tumor growth, resistance to therapies, tumor neovascularization and metastatic spread. High levels of SK1 expression or activity have been associated with poor prognosis in several cancers, including those of the prostate. Recent studies using prostate cancer cell and mouse models demonstrate a significant potential for SK1-targeting therapies to synergize with the effects of docetaxel chemotherapy and radiotherapy. However, until recently the absence of clinically applicable SK1 inhibitors has limited the translation of these findings into patients. With the recent discovery that clinically approved drug fingolimod has SK1-inhibiting properties, SK1 has gained significant attention from both clinicians and the pharmaceutical industry and it is hoped that trials of newly developed SK1 inhibitors might follow soon.

A mechanistically novel, first oral therapy for multiple sclerosis: the development of fingolimod (FTY720, Gilenya)

Multiple sclerosis (MS) is a chronic autoimmune disorder affecting the central nervous system (CNS) through demyelination and neurodegeneration. Until recently, major therapeutic treatments have relied on agents requiring injection delivery. In September 2010, fingolimod/FTY720 (Gilenya, Novartis) was approved by the FDA as the first oral treatment for relapsing forms of MS. Fingolimod is a novel compound produced by chemical modification of a fungal precursor. Its active metabolite, formed by in vivo phosphorylation, modulates sphingosine 1-phosphate (S1P) receptors that are a subset of a larger family of cell-surface, G protein-coupled receptors (GPCRs) mediating the effects of bioactive lipids known as lysophospholipids. Fingolimod's mechanism of action in MS is not completely understood; however, its relevant biology indicates a fundamentally different mechanism compared to all previously approved MS therapies, with evolving research supporting both immunological and nervous system activities. This duality may herald a paradigm shift in the treatment of MS and other neurological disorders.

The influences of metabotropic receptor activation on cellular signaling and synaptic function in amacrine cells

Amacrine cells receive glutamatergic input from bipolar cells and GABAergic, glycinergic, cholinergic, and dopaminergic input from other amacrine cells. Glutamate, GABA, glycine, and acetylcholine (ACh) interact with ionotropic receptors and it is these interactions that form much of the functional circuitry in the inner retina. However, glutamate, GABA, ACh, and dopamine also activate metabotropic receptors linked to second messenger pathways that have the potential to modify the function of individual cells as well as retinal circuitry. Here, the physiological effects of activating dopamine receptors, metabotropic glutamate receptors, GABAB receptors, and muscarinic ACh receptors on amacrine cells will be discussed. The retina also expresses metabotropic receptors and the biochemical machinery associated with the synthesis and degradation of endocannabinoids and sphingosine-1-phosphate (S1P). The effects of activating cannabinoid receptors and S1P receptors on amacrine cell function will also be addressed.

Mechanisms of fingolimod's efficacy and adverse effects in multiple sclerosis

Until recently, all approved multiple sclerosis (MS) disease treatments were administered parenterally. Oral fingolimod was approved in September 2010 by the US Food and Drug Administration to reduce relapses and disability progression in relapsing forms of MS. In the clinical trials that led to approval, fingolimod reduced not only acute relapses and magnetic resonance imaging lesion activity but also disability progression and brain volume loss, suggesting preservation of tissue. Fingolimod's mechanism of action in MS is not known with certainty. Its active form, fingolimod-phosphate (fingolimod-P), is a sphingosine 1-phosphate receptor (S1PR) modulator that inhibits egress of lymphocytes from lymph nodes and their recirculation, potentially reducing trafficking of pathogenic cells into the central nervous system (CNS). Fingolimod also readily penetrates the CNS, and fingolimod-P formed in situ may have direct effects on neural cells. Fingolimod potently inhibits the MS animal model, experimental autoimmune encephalomyelitis, but is ineffective in mice with selective deficiency of the S1P₁ S1PR subtype on astrocytes despite normal expression in the immune compartment. These findings suggest that S1PR modulation by fingolimod in both the immune system and CNS, producing a combination of beneficial anti-inflammatory and possibly neuroprotective/reparative effects, may contribute to its efficacy in MS. In clinical trials, fingolimod was generally safe and well tolerated. Its interaction with S1PRs in a variety of tissues largely accounts for the reported adverse effects, which were seen more frequently with doses 2.5 to 10x the approved 0.5 mg dose. Fingolimod's unique mechanism of action distinguishes it from all other currently approved MS therapies.

Expression of sphingosine kinase 1 in amoeboid microglial cells in the corpus callosum of postnatal rats

Sphingosine kinase 1 (SphK1), a key enzyme responsible for phosphorylating sphingosine into sphingosine-1-phosphate (S1P) has been shown to be expressed in monocytes and monocyte-derived peripheral macrophages. This study demonstrates SphK1 immunoexpression in amoeboid microglial cells (AMC), a nascent monocyte-derived brain macrophage in the corpus callosum of developing rat brain. SphK1 immunofluorescence expression, which appeared to be weak in AMC in normal brain, was markedly induced by lipopolysaccharide (LPS) or hypoxia treatment. Western blot analysis also showed increased expression level of SphK1 in the corpus callosum rich in AMC after LPS treatment. Detection of SphK1 mRNA and its upregulation after LPS treatment was confirmed in primary culture AMC by RT-PCR. Administration of N, N-dimethylsphingosine (DMS), a specific inhibitor of SphK1, effectively reduced upregulated SphK1 immunoexpression in AMC both in vivo and in vitro. This was corroborated by western blot which showed a decrease in SphK1 protein level of callosal tissue with DMS pretreatment. Remarkably, LPS-induced upregulation of the transcription factor NFκB was suppressed by DMS. We conclude that SphK1 expression in AMC may be linked to regulation of proinflammatory cytokines via an NFκB signaling pathway.

Keratinocyte secretion of cyclophilin B via the constitutive pathway is regulated through its cyclosporin-binding site

Cyclophilin B (CypB) is an endoplasmic reticulum (ER)-resident member of the cyclophilin family of proteins that bind cyclosporin A (CsA). We report that as in other cell types, CypB trafficked from the ER and was secreted by keratinocytes into the media in response to CsA. Concentrations as low as 1 pM of CsA induced secretion of CypB. Using brefeldin A, we showed that CypB is secreted from keratinocytes via the constitutive secretory pathway. We defined that substitution of tryptophan residue 128 in the CsA-binding site of CypB with alanine resulted in dissociation of CypB(W128A)-green fluorescent protein (GFP) from the ER. Photobleaching studies revealed a significant reduction in the diffusible mobility of CypB(W128A)-GFP compared with CypB(WT)-GFP, consistent with redistribution of CypB(W128A)-GFP into secretory vesicles disconnected from the ER/Golgi network. Furthermore, CsA significantly decreased the mobility of CypB(WT)-GFP but not CypB(W128A)-GFP. These studies demonstrate that therapeutically relevant concentrations of CsA regulate secretion of CypB by keratinocytes, and that a key residue within the CsA-binding site of CypB controls retention of CypB within the ER and regulates entry into the secretory pathway. As keratinocytes express CypB receptors (CD147) and CypB exhibits chemotactic properties, these data have implications for the therapeutic effects of CsA in inflammatory skin disease.

Protein S protects neurons from excitotoxic injury by activating the TAM receptor Tyro3-phosphatidylinositol 3-kinase-Akt pathway through its sex hormone-binding globulin-like region

The anticoagulant factor protein S (PS) protects neurons from hypoxic/ischemic injury. However, molecular mechanisms mediating PS protection in injured neurons remain unknown. Here, we show mouse recombinant PS protects dose-dependently mouse cortical neurons from excitotoxic NMDA-mediated neuritic bead formation and apoptosis by activating the phosphatidylinositol 3-kinase (PI3K)-Akt pathway (EC(50) = 26 ± 4 nm). PS stimulated phosphorylation of Bad and Mdm2, two downstream targets of Akt, which in neurons subjected to pathological overstimulation of NMDA receptors (NMDARs) increased the antiapoptotic Bcl-2 and Bcl-X(L) levels and reduced the proapoptotic p53 and Bax levels. Adenoviral transduction with a kinase-deficient Akt mutant (Ad.Akt(K179A)) resulted in loss of PS-mediated neuronal protection, Akt activation, and Bad and Mdm2 phosphorylation. Using the TAM receptors tyrosine kinases Tyro3-, Axl-, and Mer-deficient neurons, we showed that PS protected neurons lacking Axl and Mer, but not Tyro3, suggesting a requirement of Tyro3 for PS-mediated protection. Consistent with these results, PS dose-dependently phosphorylated Tyro3 on neurons (EC(50) = 25 ± 3 nm). In an in vivo model of NMDA-induced excitotoxic lesions in the striatum, PS dose-dependently reduced the lesion volume in control mice (EC(50) = 22 ± 2 nm) and protected Axl(-/-) and Mer(-/-) transgenic mice, but not Tyro3(-/-) transgenic mice. Using different structural PS analogs, we demonstrated that the C terminus sex hormone-binding globulin-like (SHBG) domain of PS is critical for neuronal protection in vitro and in vivo. Thus, our data show that PS protects neurons by activating the Tyro3-PI3K-Akt pathway via its SHGB domain, suggesting potentially a novel neuroprotective approach for acute brain injury and chronic neurodegenerative disorders associated with excessive activation of NMDARs.

Regulation of synaptic strength by sphingosine 1-phosphate in the hippocampus

Although the hippocampus is a brain region involved in short-term memory, the molecular mechanisms underlying memory formation are not completely understood. Here we show that sphingosine 1-phosphate (S1P) plays a pivotal role in the formation of memory. Addition of S1P to rat hippocampal slices increased the rate of AMPA receptor-mediated miniature excitatory postsynaptic currents (mEPSCs) recorded from the CA3 region of the hippocampus. In addition long-term potentiation (LTP) observed in the CA3 region was potently inhibited by a sphingosine kinase (SphK) inhibitor and this inhibition was fully reversed by S1P. LTP was impaired in hippocampal slices specifically in the CA3 region obtained from SphK1-knockout mice, which correlates well with the poor performance of these animals in the Morris water maze test. These results strongly suggest that SphK/S1P receptor signaling plays an important role in excitatory synaptic transmission in the CA3 region of hippocampus and has profound effects on hippocampal function such as spatial learning.

The sphingosine 1-phosphate receptor, S1PR₁, plays a prominent but not exclusive role in enhancing the excitability of sensory neurons

Sphingosine 1-phosphate (S1P) through its interaction with a family of G protein-coupled receptors (S1PR) is proving to have a significant impact on the activation of a variety of cell types, most notably those cells mediating the inflammatory response. Previously, we showed that S1P enhanced the excitability of small diameter sensory neurons, and mRNA for S1PR(1-4) was expressed in sensory neurons. These initial findings did not determine which S1PR subtype(s) mediated the increased excitability. Here, we report that exposure to the selective S1PR(1) agonist, SEW2871, produced a significant increase in excitability of some, but not all, sensory neurons. To further examine the role of S1PR(1), neurons were treated with siRNA targeted to S1PR(1). siRNA reduced S1PR(1) protein expression by 75% and blocked the sensitization produced by SEW2871, although some neurons remained responsive to subsequent exposure to S1P. Treatment with scramble siRNA did not alter S1PR(1) expression. Recordings from siRNA- and scramble-treated neurons suggested three distinct populations based on their sensitivities to SEW2871 and S1P. Approximately 50% of the neurons exhibited a significant increase in excitability after exposure to SEW2871 and subsequent S1P produced no additional increase; ∼25% were not affected by SEW2871 but S1P significantly increased excitability; and ∼25% of the neurons were not sensitized by either SEW2871 or S1P. RT-PCR measurements obtained from single neurons showed that 50% of the small diameter neurons expressed the mRNA for S1PR(1). These results indicate that S1PR(1) plays a prominent, although not exclusive, role in mediating the enhancement of excitability produced by S1P.

Sphingolipids in neurodegeneration

Although the brain contains a high content of sphingolipids, we know relatively little about the roles that sphingolipids play in regulating neural functions. Once regarded only for their structural roles in maintaining the integrity of cellular and sub-cellular compartments, it is now apparent that many sphingolipid species are biologically active and play important roles in regulating signaling events. Recent technological and scientific advances are rapidly increasing our knowledge of the roles that sphingolipids play in regulating normal neural activity. Likewise, we are beginning to understand how perturbations in sphingolipid metabolism contribute to the pathogenesis of a variety of neurodegenerative conditions. In this special issue of NeuroMolecular Medicine, we present a series of review articles that summarize new and emerging technologies for the analysis of sphingolipids, sphingolipid metabolic pathways, and how dysfunctions in sphingolipid metabolism contribute to neurodegeneration in lysosomal storage disorders, Alzheimer's disease and Multiple Sclerosis.

Permissive role of sphingosine on calcium-dependent endocytosis in chromaffin cells

Sphingosine has been shown to modulate neurotransmitter release. Because membrane fusion and fission involve lipid metabolism, we asked here whether sphingosine had a role in regulating endocytosis. To explore this hypothesis, we monitored changes of membrane capacitance (Cm) to study the effects of intracellular sphingosine on membrane retrieval after chromaffin cell stimulation with depolarising pulses (DPs). We found that: (1) sphingosine dialysis through the patch-clamp pipette (SpD) using the whole-cell configuration of the patch-clamp technique (WCC) favours the appearance of a pronounced endocytotic response; (2) SpD-elicited endocytosis was Ca(2+)-dependent but Ba(2+) did not substitute Ca(2+); (3) under WCC, such endocytotic response disappeared with repetitive DPs; (4) in cells preincubated with sphingomyelinase to augment endogenous sphingosine synthesis, and then voltage-clamped under the perforated-patch configuration of the patch-clamp technique (PPC), endocytosis decayed little with repeated stimulation; (5) sphingosine-1-phosphate (S1P), a metabolite of sphingosine, had a meagre effect on endocytosis; and (6) neither dynamin inhibitor dynasore nor calmodulin blocker calmidazolium affected the sphingosine elicited endocytosis. We believe this is the first report showing that sphingosine plays a permissive role in activating Ca(2+)-dependent endocytosis during cell depolarisation. This effect requires high subplasmalemmal cytosolic Ca(2+) concentrations and a cytosolic factor(s) that is dialysed with the pipette solution. Independence of dynamin and calmodulin suggests that sphingosine-dependent endocytosis could be a novel, more direct pathway for vesicle recycling under mild depolarisation stimuli.

Phosphorylation of 14-3-3ζ at serine 58 and neurodegeneration following kainic acid-induced excitotoxicity

Oxidative stress-induced cell death leads to phosphorylation of 14-3-3ζ at serine 58. 14-3-3ζ is detected at significant levels in cerebrospinal fluid after kainic acid (KA)-induced seizures. Here we examined temporal changes in 14-3-3ζ phosphorylation in the hippocampus and amygdala of mice after KA treatment. Mice were killed at 2, 6, 24, or 48 h after KA (30 mg/kg) injection. We observed an increase in TUNEL and Fluoro-Jade B (FJB)-stained neurons in the hippocampus and amygdala of KA-treated mice. Phospho (p)-14-3-3ζ and p-JNK expression was increased in the hippocampus 2 and 6 h after KA treatment, respectively. In immunohistochemical analysis, p-14-3-3ζ-positive cells were present in the CA3 region of the hippocampus and the central nucleus of amygdala (CeA) of KA-treated mice. Thus, phosphorylation of 14-3-3ζ at serine 58 may play an important role in KA-induced hippocampal and amygdaloid neuronal damage.

Cellular localization of sphingosine-1-phosphate receptor 1 expression in the human central nervous system

Sphingosine-1-phosphate (S1P), a potent lipid mediator, transduces intracellular signals through the activation of S1P receptors (S1PRs). Although S1PRs have been shown to play an important role in the central nervous system (CNS), accurate localization and the function of S1PR1 in the human CNS are still unclear. In this study, we investigated the localization of S1PR1 in the human CNS of postmortem samples, using a rabbit polyclonal antibody, the specificity of which had been well defined. Immunohistochemical investigation of paraffin-embedded sections revealed diffuse granular staining of the gray matter. The signals of the gray matter were much stronger than those of the white matter. The immunohistochemical expression levels correlated well with the results of quantitative real-time RT-PCR-based analysis and Western blotting. Studies using double immunostaining and immunoelectron microscopy revealed that the antigen was strongly expressed in the membrane of the astrocytic foot processes of glia limitans and astrocytes with radial cytoplasm, but not distributed in neurons. In neurological disorders, hypertrophic astrocytes with strong expression of glial fibrillary acidic protein exhibited significantly decreased expression of S1PR1 in contrast to its strong expression in astrocytes forming fibrillary gliosis. These results indicate that S1PR1 is localized in astrocytes, and its expression level may change during the processes that occur after brain damage.

Roles for dysfunctional sphingolipid metabolism in Alzheimer's disease neuropathogenesis

Sphingolipids in the membranes of neurons play important roles in signal transduction, either by modulating the localization and activation of membrane-associated receptors or by acting as precursors of bioactive lipid mediators. Activation of cytokine and neurotrophic factor receptors coupled to sphingomyelinases results in the generation of ceramides and gangliosides, which in turn, modify the structural and functional plasticity of neurons. In aging and neurodegenerative conditions such as Alzheimer's disease (AD), there are increased membrane-associated oxidative stress and excessive production and accumulation of ceramides. Studies of brain tissue samples from human subjects, and of experimental models of the diseases, suggest that perturbed sphingomyelin metabolism is a pivotal event in the dysfunction and degeneration of neurons that occurs in AD and HIV dementia. Dietary and pharmacological interventions that target sphingolipid metabolism should be pursued for the prevention and treatment of neurodegenerative disorders.

Plasma membrane sphingomyelin hydrolysis increases hippocampal neuron excitability by sphingosine-1-phosphate mediated mechanisms

Proteins that control the excitability of neurons, including voltage-dependent ion channels and neurotransmitter receptors, reside in a membrane lipid environment that includes sphingomyelin, but the influence of the metabolism of this lipid on excitability is unknown. Sphingomyelin in the plasma membrane can be cleaved by neutral sphingomyelinases (nSMase) to generate ceramides and sphingosine-1-phosphate (S1P) which have been shown to play a variety of roles in cellular signaling processes. We found that application of nSMase to hippocampal slices results in a selective enhancement in the population spike amplitude, resulting in fEPSP-PS potentiation of the CA3-CA1 schaeffer collateral synapse. Single cell recordings showed that nSMase activity increases action potential frequency in CA1 neurons in a reversible manner. Additional current clamp recordings showed that nSMase reduces the slow after-hyperpolarization after a burst of action potentials. Mass spectrometry-based measurements demonstrated that nSMase activity induces a rapid increase in the levels of ceramides and S1P in cells in hippocampal slices. The ability of nSMase to increase CA1 neuron excitability was blocked by an inhibitor of sphingosine kinase, the enzyme that converts ceramide to S1P. Moreover, direct intracellular application of S1P to CA1 neurons increased action potential firing. Our findings suggest roles for sphingomyelin metabolism and S1P in the positive regulation of the excitability of hippocampal neurons.