Sphingosine 1-phosphate-primed astrocytes enhance differentiation of neuronal progenitor cells

Central nervous system demyelinating diseases: glial cells at the hub of pathology

Inflammatory demyelinating diseases (IDDs) are among the main causes of inflammatory and neurodegenerative injury of the central nervous system (CNS) in young adult patients. Of these, multiple sclerosis (MS) is the most frequent and studied, as it affects about a million people in the USA alone. The understanding of the mechanisms underlying their pathology has been advancing, although there are still no highly effective disease-modifying treatments for the progressive symptoms and disability in the late stages of disease. Among these mechanisms, the action of glial cells upon lesion and regeneration has become a prominent research topic, helped not only by the discovery of glia as targets of autoantibodies, but also by their role on CNS homeostasis and neuroinflammation. In the present article, we discuss the participation of glial cells in IDDs, as well as their association with demyelination and synaptic dysfunction throughout the course of the disease and in experimental models, with a focus on MS phenotypes. Further, we discuss the involvement of microglia and astrocytes in lesion formation and organization, remyelination, synaptic induction and pruning through different signaling pathways. We argue that evidence of the several glia-mediated mechanisms in the course of CNS demyelinating diseases supports glial cells as viable targets for therapy development.

Lipid peroxidation and sphingolipid alterations in the cerebral cortex and hypothalamus of rats fed a high-protein diet

OBJECTIVES

High-protein diets (HPDs) are widely accepted to enhance satiety and energy expenditure and thus have become a popular strategy to lose weight and facilitate muscle protein synthesis. However, long-term high-protein consumption could be linked with metabolic and clinical problems such as renal and liver dysfunctions. This study verified the effects of 8-wk high-protein ingestion on lipid peroxidation and sphingolipid metabolism in the plasma, cerebral cortex, and hypothalamus in rats.

METHODS

Immunoenzymatic and spectrophotometric methods were applied to assess oxidation-reduction (redox) biomarkers and neutral sphingomyelinase activity, whereas gas-liquid chromatography and high-performance liquid chromatography were used to examine sphingolipid levels.

RESULTS

The vast majority of HPD-related alterations was restricted to the hypothalamus. Specifically, an increased rate of lipid peroxidation (increased lipid hydroperoxides, 8-isoprostanes, and thiobarbituric acid reactive substances) associated with ceramide accumulation via the activation of de novo synthesis (decreased sphinganine), salvage pathway (decreased sphingosine), and sphingomyelin hydrolysis (decreased sphingomyelin and increased neutral sphingomyelinase activity) was noted.

CONCLUSIONS

This study showed that HPD substantially affected hypothalamic metabolic pathways, which potentially alter cerebral output signals to the peripheral tissues.

SARS-CoV-2 Infection: A Role for S1P/S1P Receptor Signaling in the Nervous System?

The recent coronavirus disease (COVID-19) is still spreading worldwide. The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the virus responsible for COVID-19, binds to its receptor angiotensin-converting enzyme 2 (ACE2), and replicates within the cells of the nasal cavity, then spreads along the airway tracts, causing mild clinical manifestations, and, in a majority of patients, a persisting loss of smell. In some individuals, SARS-CoV-2 reaches and infects several organs, including the lung, leading to severe pulmonary disease. SARS-CoV-2 induces neurological symptoms, likely contributing to morbidity and mortality through unknown mechanisms. Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid with pleiotropic properties and functions in many tissues, including the nervous system. S1P regulates neurogenesis and inflammation and it is implicated in multiple sclerosis (MS). Notably, Fingolimod (FTY720), a modulator of S1P receptors, has been approved for the treatment of MS and is being tested for COVID-19. Here, we discuss the putative role of S1P on viral infection and in the modulation of inflammation and survival in the stem cell niche of the olfactory epithelium. This could help to design therapeutic strategies based on S1P-mediated signaling to limit or overcome the host-virus interaction, virus propagation and the pathogenesis and complications involving the nervous system.

Cryopreserved astrocytes maintain biological properties: Support of neuronal survival and differentiation

BACKGROUND

Astrocytes, one of the main glial cell types, play critical roles in the central nervous system (CNS) development and function, including support of neuronal survival and differentiation, blood brain barrier formation, synapse homeostasis and injury response. Cell isolation and culture techniques have been proved to be a powerful tool to study astrocyte physiology and function. Due to financial constraints and rigid biosafety and ethics rules to use animal models, freezing techniques and the creation of cell banks emerged as alternatives to optimize the use of experimental animals. One of the main challenges, however, of these techniques is to guarantee that conserved cells keep their biological properties.

NEW METHOD

In this work, we characterized morphologically and functionally murine secondary astrocyte cultures that have been submitted to freezing/thawing procedures.

RESULTS

Morphological characterization of SAC (secondary astrocyte culture) and SFAC (secondary frozen-astrocyte culture) did not reveal significant differences on astrocyte morphology, confluence time and cell number along culture period. Functionally, SAC and SFAC did not reveal differences in their potential to support neuronal survival, maturation, neuritogenesis and synapse formation.

CONCLUSIONS

Our results suggest that murine astrocytes that are submitted to freezing/thawing procedure maintain morphological and functional characteristics when compared with non-frozen astrocytes. Thus, this methodological approach is a valuable tool for in vitro research and might allow experimental optimization and reduction of animal use.

Another win for endothelial progenitor cells: Endothelial progenitor cell-derived conditioned medium promotes proliferation and exerts neuroprotection in cultured neuronal progenitor cells

Progress in stem cell research demonstrates stem cells' potential for treating neurodegenerative diseases. Stem cells have proliferative/differentiative properties and produce a variety of paracrine factors that can potentially be used to regenerate nervous tissue. Previous studies have shown the positive regenerative effects of endothelial progenitor cells (EPCs), and thus, they may be used as a tool for regeneration. A study by Di Santo et al. explored whether EPC-derived conditioned medium (EPC-CM) promotes the survival of cultured striatal progenitor cells and attempted to find the paracrine factors and signaling pathways involved with EPC-CM's effects. The neuronal progenitor cells that were cultured with EPC-CM had much higher densities of GABA-immunoreactive (GABA-ir) neurons. It was shown that phosphatidylinositol-3-kinase/AKT and mitogen-activated protein kinase/ERK signaling pathways are involved in the proliferation of GABAergic neurons, as inhibition of these pathways decreased GABAergic densities. In addition, the results suggest that paracrine factors from EPC, both proteinaceous and lipidic, significantly elevated the viability and/or differentiation in the cultures. Importantly, it was found that EPC-CM provided neuroprotection against toxins from 3-nitropropionic acid. In sum, EPC-CM engendered proliferation and regeneration of the cultured striatal cells through paracrine factors and imparted neuroprotection. Furthermore, the effects of EPC-CM may generate a cell-free therapeutic strategy to address neurodegeneration.

Endothelial Progenitor Cells Conditioned Medium Supports Number of GABAergic Neurons and Exerts Neuroprotection in Cultured Striatal Neuronal Progenitor Cells

There is growing evidence that stem and progenitor cells exert regenerative actions by means of paracrine factors. In line with these notions, we recently demonstrated that endothelial progenitor cell (EPC)-derived conditioned medium (EPC-CM) substantially increased viability of brain microvascular cells. In the present study, we aimed at investigating whether EPC-CM supports cell survival of cultured striatal progenitor cells. For that purpose, primary cultures from fetal rat embryonic (E14) ganglionic eminence were prepared and grown for 7 days in vitro (DIV). EPC-CM was administered from DIV5–7. Treatment of the striatal cultures with EPC-CM resulted in significantly increased densities of GABA-immunoreactive (-ir) neurons. Inhibition of mitogen-activated protein kinase and phosphatidylinositol-3-kinase, but not of the ROCK pathway, significantly attenuated the EPC-CM induced increase in GABA-ir cell densities. Similar results were observed when EPC-CM was subjected to proteolytic digestion and lipid extraction. Furthermore, inhibition of translation abolished the EPC-CM induced effects. Importantly, EPC-CM displayed neuroprotection against 3-nitropropionic acid induced toxicity. These findings demonstrate that EPC-derived paracrine factors substantially promote survival and/or differentiation of cultured striatal progenitor cells involving both proteinaceous factors and lipidic factors. In sum, EPC-CM constituents might lead to a novel cell-free therapeutic strategy to challenge neuronal degeneration.

Regulatory role of sphingosine kinase and sphingosine-1-phosphate receptor signaling in progenitor/stem cells

Balanced sphingolipid signaling is important for the maintenance of homeostasis. Sphingolipids were demonstrated to function as structural components, second messengers, and regulators of cell growth and survival in normal and disease-affected tissues. Particularly, sphingosine kinase 1 (SphK1) and its product sphingosine-1-phosphate (S1P) operate as mediators and facilitators of proliferation-linked signaling. Unlimited proliferation (self-renewal) within the regulated environment is a hallmark of progenitor/stem cells that was recently associated with the S1P signaling network in vasculature, nervous, muscular, and immune systems. S1P was shown to regulate progenitor-related characteristics in normal and cancer stem cells (CSCs) via G-protein coupled receptors S1Pn (n = 1 to 5). The SphK/S1P axis is crucially involved in the regulation of embryonic development of vasculature and the nervous system, hematopoietic stem cell migration, regeneration of skeletal muscle, and development of multiple sclerosis. The ratio of the S1P receptor expression, localization, and specific S1P receptor-activated downstream effectors influenced the rate of self-renewal and should be further explored as regeneration-related targets. Considering malignant transformation, it is essential to control the level of self-renewal capacity. Proliferation of the progenitor cell should be synchronized with differentiation to provide healthy lifelong function of blood, immune systems, and replacement of damaged or dead cells. The differentiation-related role of SphK/S1P remains poorly assessed. A few pioneering investigations explored pharmacological tools that target sphingolipid signaling and can potentially confine and direct self-renewal towards normal differentiation. Further investigation is required to test the role of the SphK/S1P axis in regulation of self-renewal and differentiation.

Derivation of Functional Human Astrocytes from Cerebral Organoids

Astrocytes play a critical role in the development and homeostasis of the central nervous system (CNS). Astrocyte dysfunction results in several neurological and degenerative diseases. However, a major challenge to our understanding of astrocyte physiology and pathology is the restriction of studies to animal models, human post-mortem brain tissues, or samples obtained from invasive surgical procedures. Here, we report a protocol to generate human functional astrocytes from cerebral organoids derived from human pluripotent stem cells. The cellular isolation of cerebral organoids yielded cells that were morphologically and functionally like astrocytes. Immunolabelling and proteomic assays revealed that human organoid-derived astrocytes express the main astrocytic molecular markers, including glutamate transporters, specific enzymes and cytoskeletal proteins. We found that organoid-derived astrocytes strongly supported neuronal survival and neurite outgrowth and responded to ATP through transient calcium wave elevations, which are hallmarks of astrocyte physiology. Additionally, these astrocytes presented similar functional pathways to those isolated from adult human cortex by surgical procedures. This is the first study to provide proteomic and functional analyses of astrocytes isolated from human cerebral organoids. The isolation of these astrocytes holds great potential for the investigation of developmental and evolutionary features of the human brain and provides a useful approach to drug screening and neurodegenerative disease modelling.

Effects of FTY720 (Fingolimod) on Proliferation, Differentiation, and Migration of Brain-Derived Neural Stem Cells

Insufficient proliferation, differentiation, and migration are the main pitfalls of neural stem cells (NSCs) in reparative therapeutics for the central nervous system (CNS) diseases. The potent lipid mediator sphingosine-1-phosphate (S1P) regulates cells' biological behavior broadly in the CNS. However, the effects of activating S1P on NSCs are not quite clear. In the current study, FTY720 (Fingolimod), an analog of S1P, was employed to induce the proliferation, differentiation, and migration of cultured brain-derived NSCs. The results indicated that proliferation and migration ability of NSCs were promoted by FTY720. Though we observed no obvious neuron prefers differentiation of NSCs, there were more protoplasmic astrocytes developed in the presence of certain concentration of FTY720. This work gives more comprehensive understanding of how FTY720 affects NSCs.

Sphingosine-1-phosphate receptor antagonism enhances proliferation and migration of engrafted neural progenitor cells in a model of viral-induced demyelination

The oral drug FTY720 affects sphingosine-1-phosphate (S1P) signaling on targeted cells that bear the S1P receptors S1P1, S1P3, S1P4, and S1P5. We examined the effect of FTY720 treatment on the biology of mouse neural progenitor cells (NPCs) after transplantation in a viral model of demyelination. Intracerebral infection with the neurotropic JHM strain of mouse hepatitis virus (JHMV) resulted in an acute encephalomyelitis, followed by demyelination similar in pathology to the human demyelinating disease, multiple sclerosis. We have previously reported that intraspinal transplantation of mouse NPCs into JHMV-infected animals resulted in selective colonization of demyelinated lesions, preferential differentiation into oligodendroglia accompanied by axonal preservation, and increased remyelination. Cultured NPCs expressed transcripts for S1P receptors S1P1, S1P2, S1P3, S1P4, and S1P5. FTY720 treatment of cultured NPCs resulted in increased mitogen-activated protein kinase phosphorylation and migration after exposure to the chemokine CXCL12. Administration of FTY720 to JHMV-infected mice resulted in enhanced migration and increased proliferation of transplanted NPCs after spinal cord engraftment. FTY720 treatment did not improve clinical disease, diminish neuroinflammation or the severity of demyelination, nor increase remyelination. These findings argue that FTY720 treatment selectively increases NPC proliferation and migration but does not either improve clinical outcome or enhance remyelination after transplantation into animals in which immune-mediated demyelination is initiated by the viral infection of the central nervous system.

LPA-primed astrocytes induce axonal outgrowth of cortical progenitors by activating PKA signaling pathways and modulating extracellular matrix proteins

Lysophosphatidic acid (LPA) is one of the main membrane-derived lysophospholipids, inducing diverse cellular responses like cell proliferation, cell death inhibition, and cytoskeletal rearrangement, and thus is important in many biological processes. In the central nervous system (CNS), post-mitotic neurons release LPA extracellularly whereas astrocytes do not. Astrocytes play a key role in brain development and pathology, producing various cytokines, chemokines, growth factors, and extracellular matrix (ECM) components that act as molecular coordinators of neuron-glia communication. However, many molecular mechanisms underlying these events remain unclear-in particular, how the multifaceted interplay between the signaling pathways regulated by lysophospholipids is integrated in the complex nature of the CNS. Previously we showed that LPA-primed astrocytes induce neuronal commitment by activating LPA1-LPA2 receptors. Further, we revealed that these events were mediated by modulation and organization of laminin levels by astrocytes, through the induction of the epidermal growth factor receptor (EGFR) signaling pathway and the activation of the mitogen-activated protein (MAP) kinase (MAPK) cascade in response to LPA (Spohr et al., 2008, 2011). In the present work, we aimed to answer whether LPA affects astrocytic production and rearrangement of fibronectin, and to investigate the mechanisms involved in neuronal differentiation and maturation of cortical neurons induced by LPA-primed astrocytes. We show that PKA activation is required for LPA-primed astrocytes to induce neurite outgrowth and neuronal maturation and to rearrange and enhance the production of fibronectin and laminin. We propose a potential mechanism by which neurons and astrocytes communicate, as well as how such interactions drive cellular events such as neurite outgrowth, cell fate commitment, and maturation.

An update on the biology of sphingosine 1-phosphate receptors

Sphingosine 1-phosphate (S1P) is a membrane-derived lysophospholipid that acts primarily as an ex-tracellular signaling molecule. Signals initiated by S1P are transduced by five G protein-coupled receptors, named S1P1-5 Cellular and temporal expression of the S1P receptors (S1PRs) determine their specific roles in various organ systems, but they are particularly critical for regulation of the cardiovascular, immune, and nervous systems, with the most well-known contributions of S1PR signaling being modulation of vascular barrier function, vascular tone, and regulation of lymphocyte trafficking. However, our knowledge of S1PR biology is rapidly increasing as they become attractive therapeutic targets in several diseases, such as chronic inflammatory pathologies, autoimmunity, and cancer. Understanding how the S1PRs regulate interactions between biological systems will allow for greater efficacy in this novel therapeutic strategy as well as characterization of complex physiological networks. Because of the rapidly expanding body of research, this review will focus on the most recent advances in S1PRs.

Neuron-astroglial interactions in cell-fate commitment and maturation in the central nervous system

Neuron-astroglia interactions play a key role in several events of brain development, such as neuronal generation, migration, survival, and differentiation; axonal growth; and synapse formation and function. While there is compelling evidence of the effects of astrocyte factors on neurons, their effects on astrocytes have not been fully determined. In this review, we will focus on the role of neurons in astrocyte generation and maturation. Further, we highlight the great heterogeneity and diversity of astroglial and neural progenitors such as radial glia cells, and discuss the importance of the variety of cellular interactions in controlling the structural and functional organization of the brain. Finally, we present recent data on a new role of astrocytes in neuronal maturation, as mediators of the action of biolipids in the cerebral cortex. We will argue that the functional architecture of the brain depends on an intimate neuron-glia partnership, by briefly discussing the emerging view of how neuron-astrocyte dysfunctions might be associated with neurodegenerative diseases and neurological disorders.