Mycobacterial Infection is Promoted by Neutral Sphingomyelinase 2 Regulating a Signaling Cascade Leading to Activation of β1-Integrin

Background/Aims: Mycobacteria-induced diseases, especially tuberculosis, cause more than 1 million deaths each year, which is higher than any other single bacterial pathogen. Neutral sphingomyelinase 2 (Nsm2) has been implied in many physiological processes and diseases, but the role of Nsm2 in pathogen-host interactions and mycobacterial infections has barely been studied. Methods: We investigated the role of the Nsm2/ceramide system in systemic infection of mice and murine macrophages with Mycobacterium bovis Bacillus Calmette-Guérin (BCG) as a model for mycobacterial infection. For in vitro assays we isolated bone marrow-derived macrophages from Wildtype mice or Nsm2-heterozygous and investigated the role of Nsm2 for macrophage migration/clustering as well as the involvement of p38 mitogen-activated protein kinases (p38K), c-Jun N-terminal kinase (JNK), β1-integrin and Rac1 activity by Western blot and microscopic studies. For in vivo assays we injected mice intravenously with BCG and analyzed infected tissues for the role of Nsm2-mediated activation of β1-integrin in granuloma formation and bacterial burden. Results: Our results reveal that BCG infection of macrophages results in rapid stimulation of Nsm2. Genetic and pharmacological studies demonstrate that Nsm2 stimulates a signaling cascade via p38K and JNK to an activation of surface β1-integrin and Rac1 that leads to the formation of granuloma-like macrophages clusters in vitro and granuloma in vivo. Heterozygosity of Nsm2 in macrophages or antibody-mediated neutralization of active b1-integrin reduced macrophage clusters in vitro and granuloma formation in vivo. Most importantly, Nsm2 heterozygosity or treatment with neutralizing antibodies against β1-integrin protected mice from systemic BCG infections and chronic infections of the liver and spleen. Conclusion: The findings indicate that the Nsm2/ ceramide system plays an important role in systemic infection of mice with mycobacteria by regulating a signaling cascade via p38K, JNK, b1-integrin and Rac1

Caveolin-1 affects early mycobacterial infection and apoptosis in macrophages and mice

Tuberculosis, caused by Mycobacterium tuberculosis, remains one of the deadliest infections in humans. Because Mycobacterium bovis Bacillus Calmette-Guérin (BCG) share genetic similarities with Mycobacterium tuberculosis, it is often used as a model to elucidate the molecular mechanisms of more severe tuberculosis infection. Caveolin-1 has been implied in many physiological processes and diseases, but it's role in mycobacterial infections has barely been studied. We isolated macrophages from Wildtype or Caveolin-1 deficient mice and analyzed hallmarks of infection, such as internalization, induction of autophagy and apoptosis. For in vivo assays we intravenously injected mice with BCG and investigated tissues for bacterial load with colony-forming unit assays, bioactive lipids with mass spectrometry and changes of protein expressions by Western blotting. Our results revealed that Caveolin-1 was important for early killing of BCG infection in vivo and in vitro, controlled acid sphingomyelinase (Asm)-dependent ceramide formation, apoptosis and inflammatory cytokines upon infection with BCG. In accordance, Caveolin-1 deficient mice and macrophages showed higher bacterial burdens in the livers. The findings indicate that Caveolin-1 plays a role in infection of mice and murine macrophages with BCG, by controlling cellular apoptosis and inflammatory host response. These clues might be useful in the fight against tuberculosis

Sphingolipids in Major Depression

Major depression is one of the most common and severe diseases affecting the world's population. However, the pathogenesis of the disease remains inadequately defined. Previously, a lack of monoaminergic neurotransmitters was the focus of pathophysiological concepts; however, recent concepts focus on a alteration of neurogenesis in the hippocampus. This concept suggests that neurogenesis is decreased in major depression with a rarefication of neuronal networks and a lack of new, immature neurons in the hippocampus, events that may result in the clinical symptoms of major depression. However, molecular targets involved in the pathogenesis of major depression and, in particular, a reduction of neurogenesis, are largely unknown. We have recently discovered that an inhibition of the acid sphingomyelinase/ceramide system mediates the effects of tri- and tetracyclic antidepressants. Moreover, an accumulation of ceramide in the hippocampus results in depression-like symptoms. This suggests the acid sphingomyelinase/ceramide system is very important in the pathogenesis of major depression

Regulation of Neuronal Stem Cell Proliferation in the Hippocampus by Endothelial Ceramide

Background/Aims: Major depressive disorder is one of the most common diseases in western countries. The disease is mainly defined by its psychiatric symptoms. However, the disease has also many symptoms outside the central nervous system, in particular cardiovascular symptoms. Recent studies demonstrated that the acid sphingomyelinase/ceramide system plays an important role in the development of major depressive disorder and functions as a target of antidepressants. Methods: Here, we investigated (i) whether ceramide accumulates in endothelial cells in the neurogenetic zone of the hippocampus after glucocorticosterone-mediated stress, (ii) whether ceramide is released into the extracellular space of the hippocampus and (iii) whether extracellular ceramide inhibits neuronal proliferation. Ceramide was determined in endothelial cell culture supernatants or extracellular hippocampus extracts by a kinase assay. Endothelial ceramide in the hippocampus was analyzed by confocal microscopy of brain sections stained with Cy3-labelled anti-ceramide antibodies and FITC-Isolectin B4. Neuronal proliferation was measured by incubation of pheochromocytoma neuronal cells with culture supernatants and extracellular hippocampus extracts. Results: Treatment of cultured endothelial cells with glucocorticosterone induces a release of ceramide into the supernatant. Likewise, treatment of mice with glucocorticosterone triggers a release of ceramide into the extracellular space of the hippocampus. The release of ceramide is inhibited by concomitant treatment with the antidepressant amitriptyline, which also inhibits the activity of the acid sphingomyelinase. Studies employing confocal microscopy revealed that ceramide is formed and accumulates exclusively in endothelial cells in the hippocampus of stressed mice, a process that was again prevented by co-application of amitriptyline. Ceramide released in the culture supernatant or into the extracellular space of the hippocampus reduced proliferation of neurons in vitro. Conclusion: The data suggest a novel model for the pathogenesis of major depressive disorder, i.e. the release of ceramide-enriched microvesicles from endothelial cells that negatively affect neuronal proliferation in the hippocampus, but may also induce cardiovascular disease and other systemic symptoms of patients with major depressive disorder

Inhibition of Acid Sphingomyelinase by Antidepressants Counteracts Stress-Induced Activation of P38-Kinase in Major Depression

Background/Aims: Major depressive disorder is a common disease with serious morbidity, including increased risk of death from suicide. Major depressive disorder is treated with antidepressants. However, the molecular targets of antidepressants remained ill-defined and require further elucidation. Methods: Mice were treated with corticosterone to induce stress, amitriptyline and the p38-kinase (p38K) inhibitor SB239063 or a combination of these drugs. Phosphorylation of p38K in hippocampal neurons was determined by immunostaining with a phospho-specific antibody, neuronal proliferation using BrdU-labelling and behaviour employing a set of behavioural tests. Results: Corticosterone induced phosphorylation/activation of p38K in the hippocampus in vivo. Antidepressants reversed the effect of corticosterone on p38K activation in wildtype mice, but had no effect in acid sphingomyelinase-deficient animals. Corticosterone also reduced neurogenesis and triggered depression-like behavioural changes, effects that were prevented by pharmacological inhibition of p38K. Conclusion: Stress induces p38K phosphorylation/activation in the hippocampus and thereby reduces neurogenesis and induces depression-like symptoms, events that are prevented by antidepressants via inhibition of the acid sphingomyelinase/ceramide system

Melatonin Acts as an Antidepressant by Inhibition of the Acid Sphingomyelinase/Ceramide System

Background: Melatonin has been shown to have antidepressive effects. We tested whether melatonin inhibits the acid sphingomyelinase/ceramide system and mediates its antidepressive effects via inhibition of the acid sphingomyelinase and a reduction of ceramide in the hippocampus. Antidepressants such as amitriptyline and fluoxetine were previously shown to inhibit the acid sphingomyelinase/ceramide system, which mediates neurogenesis and behavioral changes induced by these drugs. Methods: The effect of melatonin on the activity of the acid sphingomyelinase prior to and after treatment with melatonin was determined in cultured neurons and in vivo in the hippocampus of mice by measuring the consumption of [14C] sphingomyelin. Ceramide was measured by DAG kinase assay and fluorescence microscopy of the hippocampus and of cultured neurons. Neurogenesis in the hippocampus was analyzed by in vivo labeling with bromodeoxyuridine. Behavior was assessed in standardized tests. Results: Melatonin treatment inhibited acid sphingomyelinase in vitro in cultured pheochromocytoma cells and in vivo in the hippocampus, which resulted in a reduction of ceramide in vitro and in vivo. The inhibition of the acid sphingomyelinase/ceramide system translated into increased neurogenesis in glucocorticosterone-stressed mice after treatment with melatonin, an effect that is abrogated in acid sphingomyelinase-deficient mice. Likewise, melatonin improved the depressive behavior of stressed mice, a therapeutic effect that was again absent in acid sphingomyelinase-deficient animals. Conclusion: These data indicate that the antidepressive effects of melatonin as well as the induction of neurogenesis triggered by this drug are mediated by an inhibition of the acid sphingomyelinase/ceramide system. This is the first study to identify melatonin as an inhibitor of the acid sphingomyelinase

CD95 Rapidly Clusters in Cells of Diverse Origins

We have shown that CD95-mediated cell death requires a clustering of the receptor in distinct sphingolipid-rich domains of the cell membrane (Grassmé et al., 2000, Cremesti et al., 2000). These domains form in response to acid sphingomyelinase (ASM)-induced ceramide generation. However, recent studies challenged the finding of early CD95 clustering (Algeciras-Schimnich et al., 2002). Here, six independent groups tested clustering of CD95 in diverse cell type including primary cells ex vivo and established cell lines. The studies show clustering of CD95 within seconds to minutes in all cell types tested by the different groups. In addition, clustering of CD95 was detected after stimulation of cells using three agonistic anti-CD95 antibodies (CH11, APO-1-3 and JO2), CD95 ligand and stimuli that induce an upregulation and activation of the endogenous CD95/CD95 ligand system. The data confirm our previous studies and suggest rapid, i.e., within seconds to minutes, CD95 clustering as a general phenomenon occurring in many cell types

Neutral Sphingomyelinase in Physiological and Measles Virus Induced T Cell Suppression

T cell paralysis is a main feature of measles virus (MV) induced immunosuppression. MV contact mediated activation of sphingomyelinases was found to contribute to MV interference with T cell actin reorganization. The role of these enzymes in MV-induced inhibition of T cell activation remained equally undefined as their general role in regulating immune synapse (IS) activity which relies on spatiotemporal membrane patterning. Our study for the first time reveals that transient activation of the neutral sphingomyelinase 2 (NSM2) occurs in physiological co-stimulation of primary T cells where ceramide accumulation is confined to the lamellum (where also NSM2 can be detected) and excluded from IS areas of high actin turnover. Genetic ablation of the enzyme is associated with T cell hyper-responsiveness as revealed by actin dynamics, tyrosine phosphorylation, Ca2+-mobilization and expansion indicating that NSM2 acts to suppress overshooting T cell responses. In line with its suppressive activity, exaggerated, prolonged NSM2 activation as occurring in co-stimulated T cells following MV exposure was associated with aberrant compartmentalization of ceramides, loss of spreading responses, interference with accumulation of tyrosine phosphorylated protein species and expansion. Altogether, this study for the first time reveals a role of NSM2 in physiological T cell stimulation which is dampening and can be abused by a virus, which promotes enhanced and prolonged NSM2 activation to cause pathological T cell suppression