Sphingosine-1-phosphate and its effect on glucose deprivation/glucose reload stress: from gene expression to neuronal survival

Aβ-Induced Alterations in Membrane Lipids Occur before Synaptic Loss Appears

Loss of active synapses and alterations in membrane lipids are crucial events in physiological aging as well as in neurodegenerative disorders. Both are related to the abnormal aggregation of amyloid-beta (Aβ) species, generally known as amyloidosis. There are two major known human Aβ species: Aβ_(1–40) and Aβ_(1–42). However, which of these species have more influence on active synapses and membrane lipids is still poorly understood. Additionally, the time-dependent effect of Aβ species on alterations in membrane lipids of hippocampal neurones and glial cells remains unknown. Therefore, our study contributes to a better understanding of the role of Aβ species in the loss of active synapses and the dysregulation of membrane lipids in vitro. We showed that Aβ_(1–40) or Aβ_(1–42) treatment influences membrane lipids before synaptic loss appears and that the loss of active synapses is not dependent on the Aβ species. Our lipidomic data analysis showed early changes in specific lipid classes such as sphingolipid and glycerophospholipid neurones. Our results underscore the potential role of lipids as a possible early diagnostic biomarker in amyloidosis-related disorders.

Recent advances of the function of sphingosine 1-phosphate (S1P) receptor S1P3

Known as a variety of sphingolipid metabolites capable of performing various biological activities, sphingosine 1-phosphate (S1P) is commonly found in platelets, red blood cells, neutrophils, lymph fluid, and blood, as well as other cells and body fluids. S1P comprises five receptors, namely, S1P1-S1P5, with the distribution of S1P receptors exhibiting tissue selectivity to some degree. S1P1, S1P2, and S1P3 are extensively expressed in a wide variety of different tissues. The expression of S1P4 is restricted to lymphoid and hematopoietic tissues, while S1P5 is primarily expressed in the nervous system. S1P3 plays an essential role in the pathophysiological processes related to inflammation, cell proliferation, cell migration, tumor invasion and metastasis, ischemia-reperfusion, tissue fibrosis, and vascular tone. In this paper, the relevant mechanism in the role of S1P3 is summarized.

Lipids and Alzheimer's Disease

Lipids, as the basic component of cell membranes, play an important role in human health as well as brain function. The brain is highly enriched in lipids, and disruption of lipid homeostasis is related to neurologic disorders as well as neurodegenerative diseases such as Alzheimer’s disease (AD). Aging is associated with changes in lipid composition. Alterations of fatty acids at the level of lipid rafts and cerebral lipid peroxidation were found in the early stage of AD. Genetic and environmental factors such as apolipoprotein and lipid transporter carrying status and dietary lipid content are associated with AD. Insight into the connection between lipids and AD is crucial to unraveling the metabolic aspects of this puzzling disease. Recent advances in lipid analytical methodology have led us to gain an in-depth understanding on lipids. As a result, lipidomics have becoming a hot topic of investigation in AD, in order to find biomarkers for disease prediction, diagnosis, and prevention, with the ultimate goal of discovering novel therapeutics.

Sphingosine 1-Phosphate Receptors and Metabolic Enzymes as Druggable Targets for Brain Diseases

The central nervous system is characterized by a high content of sphingolipids and by a high diversity in terms of different structures. Stage- and cell-specific sphingolipid metabolism and expression are crucial for brain development and maintenance toward adult age. On the other hand, deep dysregulation of sphingolipid metabolism, leading to altered sphingolipid pattern, is associated with the majority of neurological and neurodegenerative diseases, even those totally lacking a common etiological background. Thus, sphingolipid metabolism has always been regarded as a promising pharmacological target for the treatment of brain disorders. However, any therapeutic hypothesis applied to complex amphipathic sphingolipids, components of cellular membranes, has so far failed probably because of the high regional complexity and specificity of the different biological roles of these structures. Simpler sphingosine-based lipids, including ceramide and sphingosine 1-phosphate, are important regulators of brain homeostasis, and, thanks to the relative simplicity of their metabolic network, they seem a feasible druggable target for the treatment of brain diseases. The enzymes involved in the control of the levels of bioactive sphingoids, as well as the receptors engaged by these molecules, have increasingly allured pharmacologists and clinicians, and eventually fingolimod, a functional antagonist of sphingosine 1-phosphate receptors with immunomodulatory properties, was approved for the therapy of relapsing-remitting multiple sclerosis. Considering the importance of neuroinflammation in many other brain diseases, we would expect an extension of the use of such analogs for the treatment of other ailments in the future. Nevertheless, many aspects other than neuroinflammation are regulated by bioactive sphingoids in healthy brain and dysregulated in brain disease. In this review, we are addressing the multifaceted possibility to address the metabolism and biology of bioactive sphingosine 1-phosphate as novel targets for the development of therapeutic paradigms and the discovery of new drugs.

The Role of Ceramide and Sphingosine-1-Phosphate in Alzheimer's Disease and Other Neurodegenerative Disorders

Bioactive sphingolipids-ceramide, sphingosine, and their respective 1-phosphates (C1P and S1P)-are signaling molecules serving as intracellular second messengers. Moreover, S1P acts through G protein-coupled receptors in the plasma membrane. Accumulating evidence points to sphingolipids' engagement in brain aging and in neurodegenerative disorders such as Alzheimer's, Parkinson's, and Huntington's diseases and amyotrophic lateral sclerosis. Metabolic alterations observed in the course of neurodegeneration favor ceramide-dependent pro-apoptotic signaling, while the levels of the neuroprotective S1P are reduced. These trends are observed early in the diseases' development, suggesting causal relationship. Mechanistic evidence has shown links between altered ceramide/S1P rheostat and the production, secretion, and aggregation of amyloid β/α-synuclein as well as signaling pathways of critical importance for the pathomechanism of protein conformation diseases. Sphingolipids influence multiple aspects of Akt/protein kinase B signaling, a pathway that regulates metabolism, stress response, and Bcl-2 family proteins. The cross-talk between sphingolipids and transcription factors including NF-κB, FOXOs, and AP-1 may be also important for immune regulation and cell survival/death. Sphingolipids regulate exosomes and other secretion mechanisms that can contribute to either the spread of neurotoxic proteins between brain cells, or their clearance. Recent discoveries also suggest the importance of intracellular and exosomal pools of small regulatory RNAs in the creation of disturbed signaling environment in the diseased brain. The identified interactions of bioactive sphingolipids urge for their evaluation as potential therapeutic targets. Moreover, the early disturbances in sphingolipid metabolism may deliver easily accessible biomarkers of neurodegenerative disorders.

Phospholipid and Lipid Derivatives as Potential Neuroprotective Compounds

The worldwide demographical trend is changing towards a more elderly population. In particular, this phenomenon is increasing the number of neurodegenerative disease cases (e.g., Alzheimer’s disease) in advanced countries. Therefore, there is a fertile field for neuroprotective approaches to address this problem. A useful strategy to protect the membrane integrity of cells and reduce inflammatory processes. In this context, the neurons represent particularly vulnerable cells. Thus, a protection strategy should include their membrane preservation and improved anti-inflammatory processes. The contribution of phospholipid derivatives to this issue is crucial and many articles evidence their role in both health and disease. On the other hand, some lipids containing choline actively participate to increase the choline levels in the nervous system. It is acknowledged that the cholinergic system plays a pivotal role both in the central and in the peripheral nervous system. Neurons cannot synthesize choline, which is provided by the diet. The reuptake of ACh and its hydrolysis represent the principal source of choline. Therefore, to cover choline needs, choline-containing lipids may be used. There are different works which demonstrate their neuroprotective features This review article analyzes phospholipid and lipid derivatives that through different mechanisms are involved in these protective processes, although, sometimes the same molecules may behave as neurotoxic elements, therefore, their protective machinery should be detailed better.

Corneal Endothelial Regeneration Using Mesenchymal Stem Cells Derived from Human Umbilical Cord

Corneal blindness is the third leading cause of blindness in the world, and one of the main etiologies is dysfunction of the corneal endothelium. Current treatment of corneal endothelial disease is allogenic corneal transplantation, which is limited by the global shortage of donor corneas and immunological rejection. The corneal endothelium consists of a monolayer of cells derived from the neural crest and mesoderm. Its main function is to prevent corneal edema by tight junctions formed by zonular occludens-1 (ZO-1) and Na, K-ATPase pump function. The human umbilical cord (UC) is a rich source of mesenchymal stem cells (MSCs). UC-MSCs that have multi-lineage potential may be an accessible allogenic source. After inducing differentiation with medium containing glycogen synthase kinase (GSK) 3-β inhibitor, UC-MSCs formed polygonal corneal endothelial-like cells that functioned as tissue-engineered corneal endothelium (UTECE). Expressions of major corneal endothelial markers were confirmed by reverse transcription-polymerase chain reaction (RT-PCR) and quantitative RT-PCR (qRT-PCR). Western blotting confirmed the expression of Na,K-ATPase and PITX2, the functional and developmental markers of corneal endothelial cells. Immunohistochemistry revealed the localization of Na,K-ATPase and ZO-1 in cell-cell junctions, suggesting the presence of tight junctions. In vitro functional analysis revealed that UTECE had significantly high pump function compared with UC-MSCs. Moreover, UTECE transplanted into a rabbit model of bullous keratopathy successfully maintained corneal thickness and transparency. Our findings suggest that UTECE may be used as a source of allogenic cells for the treatment of corneal endothelial disease.

Alpha-synuclein alters differently gene expression of Sirts, PARPs and other stress response proteins: implications for neurodegenerative disorders

Alpha-synuclein (ASN) is a presynaptic protein that can easily change its conformation under different types of stress. It's assumed that ASN plays an important role in the pathogenesis of Parkinson's and Alzheimer's disease. However, the molecular mechanism of ASN toxicity has not been elucidated. This study focused on the role of extracellular ASN (eASN) in regulation of transcription of sirtuins (Sirts) and DNA-bound poly(ADP-ribose) polymerases (PARPs) - proteins crucial for cells' survival/death. Our results indicate that eASN enhanced the free radicals level, decreased mitochondria membrane potential, cells viability and activated cells' death. Concomitantly eASN activated expression of antioxidative proteins (Sod2, Gpx4, Gadd45b) and DNA-bound Parp2 and Parp3. Moreover, eASN upregulated expression of Sirt3 and Sirt5, but downregulated of Sirt1, which plays an important role in cell metabolism including Aβ precursor protein (APP) processing. eASN downregulated gene expression of APP alpha secretase (Adam10) and metalloproteinases Mmp2, Mmp10 but upregulated Mmp11. Additionally, expression and activity of pro-survival sphingosine kinase 1 (Sphk1), Akt kinase and anti-apoptotic protein Bcl2 were inhibited. Moreover, higher expression of pro-apoptotic protein Bax and enhancement of apoptotic cells' death were observed. Summarizing, eASN significantly modulates transcription of Sirts and enzymes involved in APP/Aβ metabolism and through these mechanisms eASN toxicity may be enhanced. The inhibition of Sphk1 and Akt by eASN may lead to disturbances of survival pathways. These results suggest that eASN through alteration of transcription and by inhibition of pro-survival kinases may play important pathogenic role in neurodegenerative disorders.

Omega-3 fatty acids, lipids, and apoE lipidation in Alzheimer's disease: a rationale for multi-nutrient dementia prevention

In the last decade, it has become obvious that Alzheimer's disease (AD) is closely linked to changes in lipids or lipid metabolism. One of the main pathological hallmarks of AD is amyloid-β (Aβ) deposition. Aβ is derived from sequential proteolytic processing of the amyloid precursor protein (APP). Interestingly, both, the APP and all APP secretases are transmembrane proteins that cleave APP close to and in the lipid bilayer. Moreover, apoE4 has been identified as the most prevalent genetic risk factor for AD. ApoE is the main lipoprotein in the brain, which has an abundant role in the transport of lipids and brain lipid metabolism. Several lipidomic approaches revealed changes in the lipid levels of cerebrospinal fluid or in post mortem AD brains. Here, we review the impact of apoE and lipids in AD, focusing on the major brain lipid classes, sphingomyelin, plasmalogens, gangliosides, sulfatides, DHA, and EPA, as well as on lipid signaling molecules, like ceramide and sphingosine-1-phosphate. As nutritional approaches showed limited beneficial effects in clinical studies, the opportunities of combining different supplements in multi-nutritional approaches are discussed and summarized.

Fingolimod confers neuroprotection through activation of Rac1 after experimental germinal matrix hemorrhage in rat pups

Fingolimod, a sphingosine-1-phosphate receptor (S1PR) agonist, is clinically available to treat multiple sclerosis and is showing promise in treating stroke. We investigated if fingolimod provides long-term protection from experimental neonatal germinal matrix hemorrhage (GMH), aiming to support a potential mechanism of acute fingolimod-induced protection. GMH was induced in P7 rats by infusion of collagenase (0.3 U) into the right ganglionic eminence. Animals killed at 4 weeks post-GMH received low- or high-dose fingolimod (0.25 or 1.0 mg/kg) or vehicle, and underwent neurocognitive testing before histopathological evaluation. Subsequently, a cohort of animals killed at 72 h post-GMH received 1.0 mg/kg fingolimod; the specific S1PR1 agonist, SEW2871; or fingolimod co-administered with the S1PR1/3/4 inhibitor, VPC23019, or the Rac1 inhibitor, EHT1864. All drugs were injected intraperitoneally 1, 24, and 48 h post-surgery. At 72 h post-GMH, brain water content, extravasated Evans blue dye, and hemoglobin were measured as well as the expression levels of phospho-Akt, Akt, GTP-Rac1, Total-Rac1, ZO1, occludin, and claudin-3 determined. Fingolimod significantly improved long-term neurocognitive performance and ameliorated brain tissue loss. At 72 h post-GMH, fingolimod reduced brain water content and Evans blue dye extravasation as well as reversed GMH-induced loss of tight junctional proteins. S1PR1 agonism showed similar protection, whereas S1PR or Rac1 inhibition abolished the protective effect of fingolimod. Fingolimod treatment improved functional and morphological outcomes after GMH, in part, by tempering acute post-hemorrhagic blood-brain barrier disruption via the activation of the S1PR1/Akt/Rac1 pathway.

Neurosphere formation enhances the neurogenic differentiation potential and migratory ability of umbilical cord-mesenchymal stromal cells

BACKGROUND AIMS

The human umbilical cord (UC) is a rich source of mesenchymal stromal cells (MSCs), which have been reported to have multi-lineage potential. The objectives of this study were to investigate the characteristics and capacity of UC-MSC neurosphere formation and whether this event enhances the propensity of UC-MSCs to undergo neural differentiation.

METHODS

UC-MSCs were collected by the improved explant method. UC-MSCs and neurosphere-forming UC-MSCs (UC-MSC-neurospheres) were induced to undergo neurogenic differentiation, the latter of which were induced by suspension culturing in the presence of epidermal growth factor and basic fibroblast growth factor. The differentiation and migratory capacities of the individual cultures were then compared on the basis of the expression of neural markers, as measured by immunocytochemistry, immunoblotting and quantitative real-time polymerase chain reaction and transwell assays, respectively.

RESULTS

Both UC-MSCs and UC-MSC-neurospheres were capable of differentiating into neurogenic cells when cultured in neurogenic differentiation medium. However, pre-conditioned UC-MSC-neurospheres exhibited significantly higher expression of neural markers--including microtubule-associated protein (MAP2), MUSASHI1, glial fibrillary acidic protein (GFAP), and NESTIN--compared with those derived from UC-MSCs directly. Moreover, UC-MSC-neurospheres expressed significantly higher levels of the stemness markers NANOG, KLF4 and OCT4 than did UC-MSCs. Migration assays also revealed that both UC-MSCs and UC-MSC-neurospheres actively migrate toward glucose-depleted cells.

CONCLUSIONS

Neurogenic differentiation potential probably is greater in UC-MSC-neurospheres than in UC-MSCs. Thus, UC-MSC-neurospheres may serve as a better source of cells for neurogenic regenerative medicine.