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Sphingosine 1-phosphate (S1P) is a potent vasculoprotective and neuroprotective signaling lipid, synthesized primarily by sphingosine kinase 2 (SK2) in the brain. We have reported pronounced loss of S1P and SK2 activity early in Alzheimer's disease (AD) pathogenesis, and an inverse correlation between hippocampal S1P levels and age in females, leading us to speculate that loss of S1P is a sensitizing influence for AD. Paradoxically, SK2 was reported to mediate amyloid β (Aβ) formation from amyloid precursor protein (APP) in vitro To determine whether loss of S1P sensitizes to Aβ-mediated neurodegeneration, we investigated whether SK2 deficiency worsens pathology and memory in male J20 (PDGFB-APP SwInd ) mice. SK2 deficiency greatly reduced Aβ content in J20 mice, associated with significant improvements in epileptiform activity and cross-frequency coupling measured by hippocampal electroencephalography. However, several key measures of APP SwInd -dependent neurodegeneration were enhanced on the SK2-null background, despite reduced Aβ burden. These included hippocampal volume loss, oligodendrocyte attrition and myelin loss, and impaired performance in Y-maze and social novelty memory tests. Inhibition of the endosomal cholesterol exporter NPC1 greatly reduced sphingosine phosphorylation in glial cells, linking loss of SK2 activity and S1P in AD to perturbed endosomal lipid metabolism. Our findings establish SK2 as an important endogenous regulator of both APP processing to Aβ, and oligodendrocyte survival, in vivo These results urge greater consideration of the roles played by oligodendrocyte dysfunction and altered membrane lipid metabolic flux as drivers of neurodegeneration in AD. SIGNIFICANCE STATEMENT Genetic, neuropathological, and functional studies implicate both Aβ and altered lipid metabolism and/or signaling as key pathogenic drivers of Alzheimer's disease. In this study, we first demonstrate that the enzyme SK2, which generates the signaling lipid S1P, is required for Aβ formation from APP in vivo Second, we establish a new role for SK2 in the protection of oligodendrocytes and myelin. Loss of SK2 sensitizes to Aβ-mediated neurodegeneration by attenuating oligodendrocyte survival and promoting hippocampal atrophy, despite reduced Aβ burden. Our findings support a model in which Aβ-independent sensitizing influences such as loss of neuroprotective S1P are more important drivers of neurodegeneration than gross Aβ concentration or plaque density., (Copyright © 2019 the authors.)

Remyelination failure by oligodendrocytes contributes to the functional impairment that characterizes the demyelinating disease multiple sclerosis (MS). Since incomplete remyelination will irreversibly damage axonal connections, treatments effectively promoting remyelination are pivotal in halting disease progression. Our previous findings suggest that fibronectin aggregates, as an environmental factor, contribute to remyelination failure by perturbing oligodendrocyte progenitor cell (OPC) maturation. Here, we aim at elucidating whether exogenously added gangliosides (i.e., cell surface lipids with a potential to modulate signaling pathways) could counteract fibronectin-mediated inhibition of OPC maturation. Exclusive exposure of rat oligodendrocytes to GD1a, but not other gangliosides, overcomes aggregated fibronectin-induced inhibition of myelin membrane formation, in vitro , and OPC differentiation in fibronectin aggregate containing cuprizone-induced demyelinated lesions in male mice. GD1a exerts its effect on OPCs by inducing their proliferation and, at a late stage, by modulating OPC maturation. Kinase activity profiling revealed that GD1a activated a protein kinase A (PKA)-dependent signaling pathway and increased phosphorylation of the transcription factor cAMP response element-binding protein. Consistently, the effect of GD1a in restoring myelin membrane formation in the presence of fibronectin aggregates was abolished by the PKA inhibitor H89, whereas the effect of GD1a was mimicked by the PKA activator dibutyryl-cAMP. Together, GD1a overcomes the inhibiting effect of aggregated fibronectin on OPC maturation by activating a PKA-dependent signaling pathway. Given the persistent presence of fibronectin aggregates in MS lesions, ganglioside GD1a might act as a potential novel therapeutic tool to selectively modulate the detrimental signaling environment that precludes remyelination. SIGNIFICANCE STATEMENT As an environmental factor, aggregates of the extracellular matrix protein fibronectin perturb the maturation of oligodendrocyte progenitor cells (OPCs), thereby impeding remyelination, in the demyelinating disease multiple sclerosis (MS). Here we demonstrate that exogenous addition of ganglioside GD1a overcomes the inhibiting effect of aggregated fibronectin on OPC maturation, both in vitro and in vivo , by activating a PKA-dependent signaling pathway. We propose that targeted delivery of GD1a to MS lesions may act as a potential novel molecular tool to boost maturation of resident OPCs to overcome remyelination failure and halt disease progression., (Copyright © 2017 the authors 0270-6474/17/379925-14$15.00/0.)

Multiple sclerosis (MS) is a neuroinflammatory, demyelinating disease of the CNS. Fibrinogen deposition at sites of blood-brain barrier breakdown is a prominent feature of neuroinflammatory disease and contributes to disease severity. Plasminogen, the primary fibrinolytic enzyme, also modifies inflammatory processes. We used a murine model of MS, experimental autoimmune encephalomyelitis (EAE), to evaluate the hypothesis that the loss of plasminogen would exacerbate neuroinflammatory disease. However, contrary to initial expectations, EAE-challenged plasminogen-deficient (Plg - ) mice developed significantly delayed disease onset and reduced disease severity compared with wild-type (Plg + ) mice. Similarly, pharmacologic inhibition of plasmin activation with tranexamic acid also delayed disease onset. The T-cell response to immunization was similar between genotypes, suggesting that the contribution of plasminogen was downstream of the T-cell response. Spinal cords from EAE-challenged Plg - mice demonstrated significantly decreased demyelination and microglial/macrophage accumulation compared with Plg + mice. Although fibrinogen-deficient mice or mice with combined deficiencies of plasminogen and fibrinogen had decreased EAE severity, they did not exhibit the delay in EAE disease onset, as seen in mice with plasminogen deficiency alone. Together, these data suggest that plasminogen and plasmin-mediated fibrinolysis is a key modifier of the onset of neuroinflammatory demyelination. SIGNIFICANCE STATEMENT Multiple sclerosis is a severe, chronic, demyelinating disease. Understanding the pathobiology related to the autoreactive T-cell and microglial/macrophage demyelinating response is critical to effectively target therapeutics. We describe for the first time that deficiency of plasminogen, the key fibrinolytic enzyme, delays disease onset and protects from the development of the paralysis associated with a murine model of multiple sclerosis, experimental autoimmune encephalomyelitis (EAE). Administration of a widely used, pharmacologic inhibitor of plasminogen activation, tranexamic acid, also delays the onset of neuroinflammation associated with EAE., (Copyright © 2017 the authors 0270-6474/17/373776-13$15.00/0.)

Interferon regulatory factor 1 (IRF-1) is a transcription factor that has been implicated in the pathogenesis of the human autoimmune demyelinating disease multiple sclerosis (MS) and in its animal model, experimental autoimmune encephalomyelitis (EAE). The goal of the present study was to directly examine the role of IRF-1 in oligodendrocyte injury and inflammatory demyelination. For the purpose of this study, we generated a transgenic mouse line (CNP/dnIRF-1) that overexpresses the dominant-negative form of IRF-1 (dnIRF1) specifically in oligodendrocytes. CNP/dnIRF-1 mice exhibited no phenotypic abnormalities but displayed suppressed IRF-1 signaling in oligodendrocytes. The major finding of our study was that the CNP/dnIRF-1 mice, compared with the wild-type mice, were protected against EAE, a phenomenon associated with significant reduction of inflammatory demyelination and with oligodendrocyte and axonal preservation. The observed protection was related to suppressed IRF-1 signaling and impaired expression of immune and proapoptotic genes in oligodendrocytes. No significant differences in the peripheral immune responses between the wild-type and the CNP/dnIRF-1 mice were identified throughout the experiments. This study indicates that IRF-1 plays a critical role in the pathogenesis of EAE by mediating oligodendrocyte response to inflammation and injury. It also suggests that oligodendrocytes are actively involved in the neuroimmune network, and that exploring oligodendrocyte-related pathogenic mechanisms, in addition to the conventional immune-based ones, may have important therapeutic implications in MS.

Destruction or changes associated with myelin membranes in the CNS play a key role in the pathogenesis of multiple sclerosis and other related neurodegenerative disorders. A long-standing goal has been to detect and quantify myelin content in vivo. For this reason, we have developed a myelin-imaging technique based on positron emission tomography (PET). PET is a quantitative imaging modality that has been widely used in clinical settings for direct assessment of biological processes at the molecular level. However, lack of myelin-imaging probes has hampered the use of PET for imaging of myelination in the CNS. Here, we report a myelin-imaging agent, termed Case Imaging Compound (CIC) that readily penetrates the blood-brain barrier and preferentially localizes to myelinated regions of the brain. After radiolabeling with positron-emitting carbon-11, [(11)C]CIC-PET was conducted in longitudinal studies using a lysolethicin-induced rat model of focal demyelination and subsequent remyelination. Quantitative analysis showed that the retention of [(11)C]CIC correlates with the level of demyelination/remyelination. These studies indicate that, for the first time, [(11)C]CIC-PET can be used as an imaging marker of myelination, which has the potential to be translated into clinical studies in multiple sclerosis and other myelin-related diseases for early diagnosis, subtyping, and efficacy evaluation of therapeutic treatments aimed at myelin repair.