Altered expression of sphingosine kinase 1 and sphingosine-1-phosphate receptor 1 in mouse hippocampus after kainic acid treatment

From the Molecular Mechanism to Pre-clinical Results: Anti-epileptic Effects of Fingolimod

Epilepsy is a devastating neurological condition characterized by long-term tendency to generate unprovoked seizures, affecting around 1-2% of the population worldwide. Epilepsy is a serious health concern which often associates with other neurobehavioral comorbidities that further worsen disease conditions. Despite tremendous research, the mainstream anti-epileptic drugs (AEDs) exert only symptomatic relief leading to 30% of untreatable patients. This reflects the complexity of the disease pathogenesis and urges the precise understanding of underlying mechanisms in order to explore novel therapeutic strategies that might alter the disease progression as well as minimize the epilepsy-associated comorbidities. Unfortunately, the development of novel AEDs might be a difficult process engaging huge funds, tremendous scientific efforts and stringent regulatory compliance with a possible chance of end-stage drug failure. Hence, an alternate strategy is drug repurposing, where anti-epileptic effects are elicited from drugs that are already used to treat non-epileptic disorders.

Herein, we provide evidence of the anti-epileptic effects of Fingolimod (FTY720), a modulator of sphingosine-1-phosphate (S1P) receptor, USFDA approved already for Relapsing-Remitting Multiple Sclerosis (RRMS). Emerging experimental findings suggest that Fingolimod treatment exerts disease-modifying anti-epileptic effects based on its anti-neuroinflammatory properties, potent neuroprotection, anti-gliotic effects, myelin protection, reduction of mTOR signaling pathway and activation of microglia and astrocytes. We further discuss the underlying molecular crosstalk associated with the anti-epileptic effects of Fingolimod and provide evidence for repurposing Fingolimod to overcome the limitations of current AEDs.

Spatiotemporal Expression of SphK1 and S1PR2 in the Hippocampus of Pilocarpine Rat Model and the Epileptic Foci of Temporal Lobe Epilepsy

Temporal lobe epilepsy (TLE) is a severe chronic neurological disease caused by abnormal discharge of neurons in the brain and seriously affect the long-term life quality of patients. Currently, new insights into the pathogenesis of TLE are urgently needed to provide more personalized and effective therapeutic strategies. Accumulating evidence suggests that sphingosine kinase 1 (SphK1)/sphingosine 1-phosphate receptor 2 (S1PR2) signaling pathway plays a pivotal role in central nervous system (CNS) diseases. However, the precise altered expression of SphK1 and S1PR2 in TLE is remaining obscure. Here, we have confirmed the expression of SphK1 and S1PR2 in the pilocarpine-induced epileptic rat hippocampus and report for the first time the expression of SphK1 and S1PR2 in the temporal cortex of TLE patients. We found an increased expression of SphK1 in the brain from both epileptic rats and TLE patients. Conversely, S1PR2 expression level was markedly decreased. We further investigated the localization of SphK1 and S1PR2 in epileptic brains. Our study showed that both SphK1 and S1PR2 co-localized with activated astrocytes and neurons. Surprisingly, we observed different subcellular localization of SphK1 and S1PR2 in epileptic brain specimens. Taken together, our study suggests that the alteration of the SphK1/S1PR2 signaling axis is closely associated with the course of TLE and provides a new target for the treatment of TLE.

Mechanism of Siponimod: Anti-Inflammatory and Neuroprotective Mode of Action

Multiple sclerosis (MS) is a neuroinflammatory disorder of the central nervous system (CNS), and represents one of the main causes of disability in young adults. On the histopathological level, the disease is characterized by inflammatory demyelination and diffuse neurodegeneration. Although on the surface the development of new inflammatory CNS lesions in MS may appear consistent with a primary recruitment of peripheral immune cells, questions have been raised as to whether lymphocyte and/or monocyte invasion into the brain are really at the root of inflammatory lesion development. In this review article, we discuss a less appreciated inflammation-neurodegeneration interplay, that is: Neurodegeneration can trigger the formation of new, focal inflammatory lesions. We summarize old and recent findings suggesting that new inflammatory lesions develop at sites of focal or diffuse degenerative processes within the CNS. Such a concept is discussed in the context of the EXPAND trial, showing that siponimod exerts anti-inflammatory and neuroprotective activities in secondary progressive MS patients. The verification or rejection of such a concept is vital for the development of new therapeutic strategies for progressive MS.

AQP4‑knockout alleviates the lipopolysaccharide‑induced inflammatory response in astrocytes via SPHK1/MAPK/AKT signaling

To date, aquaporin‑4 (AQP4) has been considered as a critical contributor to neuroinflammation, but little is known about the underlying mechanism. Previous studies have shown that a critical enzyme involved in the sphingomyelin cycle, sphingosine kinase 1 (SPHK1), is implicated in inflammatory processes and contributes to chronic neuroinflammation. The present study investigated the role of AQP4 in proinflammatory cytokine release from astrocytes, with an emphasis on the SPHK1/mitogen‑activated protein kinase (MAPK)/protein kinase B (AKT) pathway. Using primary cultures isolated from AQP4+/+ and AQP4‑/‑ embryos, the production of tumor necrosis factor‑α (TNF‑α)/interleukin‑6 (IL‑6) from astrocytes challenged by lipopolysaccharide (LPS) was compared. The results showed increased secretion of TNF‑α/IL‑6 in the two groups following LPS treatment, but a significantly lower level was observed in the AQP4‑/‑ group compared with that in the AQP4+/+ group. Although upregulation of SPHK1 was detected in the two genotypes, only a mild increase in SPHK1 was found in the AQP4‑/‑ genotype. The phosphorylation of MAPK/AKT was also confirmed to be attenuated in the AQP4‑/‑ group, suggesting decreased MAPK/AKT signaling over time in AQP4‑/‑ astrocytes. Overall, the study findings demonstrated that AQP4 deficiency alleviates proinflammatory cytokine release from astrocytes, in association with the SPHK1/MAPK/AKT pathway. This data improves our understanding of AQP4 in neuroinflammatory events, highlighting a novel profile of SPHK1 as a potential target for the treatment of CNS inflammation.

Activation of Sphingosine 1-Phosphate Receptor 1 Enhances Hippocampus Neurogenesis in a Rat Model of Traumatic Brain Injury: An Involvement of MEK/Erk Signaling Pathway

Among sphingosine 1-phosphate receptors (S1PRs) family, S1PR1 has been shown to be the most highly expressed subtype in neural stem cells (NSCs) and plays a crucial role in the migratory property of NSCs. Recent studies suggested that S1PR1 was expressed abundantly in the hippocampus, a specific neurogenic region in rodent brain for endogenous neurogenesis throughout life. However, the potential association between S1PR1 and neurogenesis in hippocampus following traumatic brain injury (TBI) remains unknown. In this study, the changes of hippocampal S1PR1 expression after TBI and their effects on neurogenesis and neurocognitive function were investigated, focusing on particularly the extracellular signal-regulated kinase (Erk) signaling pathway which had been found to regulate multiple properties of NSCs. The results showed that a marked upregulation of S1PR1 occurred with a peak at 7 days after trauma, revealing an enhancement of proliferation and neuronal differentiation of NSCs in hippocampus due to S1PR1 activation. More importantly, it was suggested that mitogen-activated protein kinase-Erk kinase (MEK)/Erk cascade was required for S1PR1-meidated neurogenesis and neurocognitive recovery following TBI. This study lays a preliminary foundation for future research on promoting hippocampal neurogenesis and improving TBI outcome.

An enriched environment improves cognitive performance in mice from the senescence-accelerated prone mouse 8 strain: Role of upregulated neurotrophic factor expression in the hippocampus

In this study, we examined 3-month-old female mice from the senescence-accelerated prone mouse 8 strain and age-matched homologous normal aging female mice from the senescence accelerated- resistant mouse 1 strain. Mice from each strain were housed in an enriched environment (including a platform, running wheels, tunnel, and some toys) or a standard environment for 3 months. The mice housed in the enriched environment exhibited shorter escape latencies and a greater percentage of time in the target quadrant in the Morris water maze test, and they exhibited reduced errors and longer latencies in step-down avoidance experiments compared with mice housed in the standard environment. Correspondently, brain-derived neurotrophic factor mRNA and protein expression in the hippocampus was significantly higher in mice housed in the enriched environment compared with those housed in the standard environment, and the level of hippocampal brain-derived neurotrophic factor protein was positively correlated with the learning and memory abilities of mice from the senescence-accelerated prone mouse 8 strain. These results suggest that an enriched environment improved cognitive performance in mice form the senescence-accelerated prone mouse 8 strain by increasing brain-derived neurotrophic factor expression in the hippocampus.

Involvement of sphingolipids in ethanol neurotoxicity in the developing brain

Ethanol-induced neuronal death during a sensitive period of brain development is considered one of the significant causes of fetal alcohol spectrum disorders (FASD). In rodent models, ethanol triggers robust apoptotic neurodegeneration during a period of active synaptogenesis that occurs around the first two postnatal weeks, equivalent to the third trimester in human fetuses. The ethanol-induced apoptosis is mitochondria-dependent, involving Bax and caspase-3 activation. Such apoptotic pathways are often mediated by sphingolipids, a class of bioactive lipids ubiquitously present in eukaryotic cellular membranes. While the central role of lipids in ethanol liver toxicity is well recognized, the involvement of sphingolipids in ethanol neurotoxicity is less explored despite mounting evidence of their importance in neuronal apoptosis. Nevertheless, recent studies indicate that ethanol-induced neuronal apoptosis in animal models of FASD is mediated or regulated by cellular sphingolipids, including via the pro-apoptotic action of ceramide and through the neuroprotective action of GM1 ganglioside. Such sphingolipid involvement in ethanol neurotoxicity in the developing brain may provide unique targets for therapeutic applications against FASD. Here we summarize findings describing the involvement of sphingolipids in ethanol-induced apoptosis and discuss the possibility that the combined action of various sphingolipids in mitochondria may control neuronal cell fate.

Ethanol triggers sphingosine 1-phosphate elevation along with neuroapoptosis in the developing mouse brain

Our previous studies have indicated that de novo ceramide synthesis plays a critical role in ethanol-induced apoptotic neurodegeneration in the 7-day-old mouse brain. In this study, we examined whether the formation of sphingosine 1-phosphate (S1P), a ceramide metabolite, is associated with this apoptotic pathway. Analyses of basal levels of S1P-related compounds indicated that S1P, sphingosine, sphingosine kinase 2, and S1P receptor 1 increased significantly during postnatal brain development. In the 7-day-old mouse brain, sphingosine kinase 2 was localized mainly in neurons. Subcellular fractionation studies of the brain homogenates showed that sphingosine kinase 2 was enriched in the plasma membrane and the synaptic membrane/synaptic vesicle fractions, but not in the nuclear and mitochondrial/lysosomal fractions. Ethanol exposure in 7-day-old mice induced sphingosine kinase 2 activation and increased the brain level of S1P transiently 2-4 h after exposure, followed by caspase 3 activation that peaked around 8 h after exposure. Treatment with dimethylsphingosine, an inhibitor of sphingosine kinases, attenuated the ethanol-induced caspase 3 activation and the subsequent neurodegeneration. These results indicate that ethanol activates sphingosine kinase 2, leading to a transient increase in S1P, which may be involved in neuroapoptotic action of ethanol in the developing brain.

Sphingosine kinase 1 and sphingosine 1-phosphate receptor 3 are functionally upregulated on astrocytes under pro-inflammatory conditions

BACKGROUND

Reactive astrocytes are implicated in the development and maintenance of neuroinflammation in the demyelinating disease multiple sclerosis (MS). The sphingosine kinase 1 (SphK1)/sphingosine1-phosphate (S1P) receptor signaling pathway is involved in modulation of the inflammatory response in many cell types, but the role of S1P receptor subtype 3 (S1P(3)) signaling and SphK1 in activated rat astrocytes has not been defined.

METHODOLOGY/PRINCIPAL FINDINGS

Using immunohistochemistry we observed the upregulation of S1P(3) and SphK1 expression on reactive astrocytes and SphK1 on macrophages in MS lesions. Increased mRNA and protein expression of S1P(3) and SphK1, as measured by qPCR and Western blotting respectively, was observed after treatment of rat primary astrocyte cultures with the pro-inflammatory stimulus lipopolysaccharide (LPS). Activation of SphK by LPS stimulation was confirmed by SphK activity assay and was blocked by the use of the SphK inhibitor SKI (2-(p-hydroxyanilino)-4-(p-chlorphenyl) thiazole. Treatment of astrocytes with a selective S1P(3) agonist led to increased phosphorylation of extracellular signal-regulated kinase (ERK)-1/2), which was further elevated with a LPS pre-challenge, suggesting that S1P(3) upregulation can lead to increased functionality. Moreover, astrocyte migration in a scratch assay was induced by S1P and LPS and this LPS-induced migration was sensitive to inhibition of SphK1, and independent of cell proliferation. In addition, S1P induced secretion of the potentially neuroprotective chemokine CXCL1, which was increased when astrocytes were pre-challenged with LPS. A more prominent role of S1P(3) signaling compared to S1P(1) signaling was demonstrated by the use of selective S1P(3) or S1P(1) agonists.

CONCLUSION/SIGNIFICANCE

In summary, our data demonstrate that the SphK1/S1P(3) signaling axis is upregulated when astrocytes are activated by LPS. This signaling pathway appears to play a role in the establishment and maintenance of astrocyte activation. Upregulation of the pathway in MS may be detrimental, e.g. through enhancing astrogliosis, or beneficial through increased remyelination via CXCL1.

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.