Acid sphingomyelinase deficiency increases susceptibility to fatal alphavirus encephalomyelitis

Sphingomyelinases in a journey to combat arthropod-borne pathogen transmission

Ixodes scapularis ticks feed on humans and other vertebrate hosts and transmit several pathogens of public health concern. Tick saliva is a complex mixture of bioactive proteins, lipids and immunomodulators, such as I. scapularis sphingomyelinase (IsSMase)-like protein, an ortholog of dermonecrotoxin SMase D found in the venom of Loxosceles spp. of spiders. IsSMase modulates the host immune response towards Th2, which suppresses Th1-mediated cytokines to facilitate pathogen transmission. Arboviruses utilize exosomes for their transmission from tick to the vertebrate host, and exosomes derived from tick saliva/salivary glands suppress C-X-C motif chemokine ligand 12 and interleukin-8 immune response(s) in human skin to delay wound healing and repair processes. IsSMase affects also viral replication and exosome biogenesis, thereby inhibiting tick-to-vertebrate host transmission of pathogenic exosomes. In this review, we elaborate on exosomes and their biogenesis as potential candidates for developing novel control measure(s) to combat tick-borne diseases. Such targets could help with the development of an efficient anti-tick vaccine for preventing the transmission of tick-borne pathogens.

The Anti-Infectious Role of Sphingosine in Microbial Diseases

Sphingolipids are important structural membrane components and, together with cholesterol, are often organized in lipid rafts, where they act as signaling molecules in many cellular functions. They play crucial roles in regulating pathobiological processes, such as cancer, inflammation, and infectious diseases. The bioactive metabolites ceramide, sphingosine-1-phosphate, and sphingosine have been shown to be involved in the pathogenesis of several microbes. In contrast to ceramide, which often promotes bacterial and viral infections (for instance, by mediating adhesion and internalization), sphingosine, which is released from ceramide by the activity of ceramidases, kills many bacterial, viral, and fungal pathogens. In particular, sphingosine is an important natural component of the defense against bacterial pathogens in the respiratory tract. Pathologically reduced sphingosine levels in cystic fibrosis airway epithelial cells are normalized by inhalation of sphingosine, and coating plastic implants with sphingosine prevents bacterial infections. Pretreatment of cells with exogenous sphingosine also prevents the viral spike protein of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) from interacting with host cell receptors and inhibits the propagation of herpes simplex virus type 1 (HSV-1) in macrophages. Recent examinations reveal that the bactericidal effect of sphingosine might be due to bacterial membrane permeabilization and the subsequent death of the bacteria.

Keep Your Friends Close, but Your Enemies Closer: Role of Acid Sphingomyelinase During Infection and Host Response

Breakdown of the inert and constitutive membrane building block sphingomyelin to the highly active lipid mediator ceramide by extracellularly active acid sphingomyelinase is tightly regulated during stress response and opens the gate for invading pathogens, triggering the immune response, development of remote organ failure, and tissue repair following severe infection. How do one enzyme and one mediator manage all of these affairs? Under physiological conditions, the enzyme is located in the lysosomes and takes part in the noiseless metabolism of sphingolipids, but following stress the protein is secreted into circulation. When secreted, acid sphingomyelinase (ASM) is able to hydrolyze sphingomyelin present at the outer leaflet of membranes to ceramide. Its generation troubles the biophysical context of cellular membranes resulting in functional assembly and reorganization of proteins and receptors, also embedded in highly conserved response mechanisms. As a consequence of cellular signaling, not only induction of cell death but also proliferation, differentiation, and fibrogenesis are affected. Here, we discuss the current state of the art on both the impact and function of the enzyme during host response and damage control. Also, the potential role of lysosomotropic agents as functional inhibitors of this upstream alarming cascade is highlighted.

Tick-Borne Flavivirus Inhibits Sphingomyelinase (IsSMase), a Venomous Spider Ortholog to Increase Sphingomyelin Lipid Levels for Its Survival in Ixodes scapularis Ticks

Our previous study showed that cells from medically important arthropods, such as ticks, secrete extracellular vesicles (EVs) including exosomes that mediate transmission of flavivirus RNA and proteins to the human cells. Understanding the molecular determinants and mechanism(s) of arthropod-borne flavivirus transmission via exosome biogenesis is very important. In this current study, we showed that in the presence of tick-borne Langat Virus (LGTV; a member of tick-borne encephalitis virus complex), the expression of arthropod IsSMase, a sphingomyelinase D (SMase D) that catalyzes the hydrolytic cleavage of substrates like sphingomyelin (SM) lipids, was significantly reduced in both Ixodes scapularis ticks (in vivo) and in tick cells (in vitro). The IsSMase reduced levels correlated with down-regulation of its activity upon LGTV replication in tick cells. Our data show that LGTV-mediated suppression of IsSMase allowed accumulation of SM lipid levels that supported membrane-associated viral replication and exosome biogenesis. Inhibition of viral loads and SM lipid built up upon GW4869 inhibitor treatment reversed the IsSMase levels and restored its activity. Our results suggest an important role for this spider venomous ortholog IsSMase in regulating viral replication associated with membrane-bound SM lipids in ticks. In summary, our study not only suggests a novel role for arthropod IsSMase in tick-LGTV interactions but also provides new insights into its important function in vector defense mechanism(s) against tick-borne virus infection and in anti-viral pathway(s).

Depletion of Host and Viral Sphingomyelin Impairs Influenza Virus Infection

Influenza A virus (IAV) is a major human respiratory pathogen causing annual epidemics as well as periodic pandemics. A complete understanding of the virus pathogenesis and host factors involved in the viral lifecycle is crucial for developing novel therapeutic approaches. Sphingomyelin (SM) is the most abundant membrane sphingolipid. It preferentially associates with cholesterol to form distinct domains named lipid rafts. Sphingomyelinases, including acid sphingomyelinase (ASMase), catalyzes the hydrolysis of membrane SM and consequently transform lipid rafts into ceramide-enriched membrane platforms. In this study, we investigated the effect of SM hydrolysis on IAV propagation. Depleting plasma membrane SM by exogenous bacterial SMase (bSMase) impaired virus infection and reduced virus entry, whereas exogenous SM enhanced infection. Moreover, the depletion of virus envelope SM also reduced virus infectivity and impaired its attachment and internalization. Nonetheless, inhibition of ASMase by desipramine did not affect IAV infection. Similarly, virus replication was not impaired in Niemann-Pick disease type A (NPA) cells, which lack functional ASMase. IAV infection in A549 cells was associated with suppression of ASMase activity starting at 6 h post-infection. Our data reveals that intact cellular and viral envelope SM is required for efficient IAV infection. Therefore, SM metabolism can be a potential target for therapeutic intervention against influenza virus infection.

Use of Acid Ceramidase and Sphingosine Kinase Inhibitors as Antiviral Compounds Against Measles Virus Infection of Lymphocytes in vitro

As structural membrane components and signaling effector molecules sphingolipids influence a plethora of host cell functions, and by doing so also the replication of viruses. Investigating the effects of various inhibitors of sphingolipid metabolism in primary human peripheral blood lymphocytes (PBL) and the human B cell line BJAB we found that not only the sphingosine kinase (SphK) inhibitor SKI-II, but also the acid ceramidase inhibitor ceranib-2 efficiently inhibited measles virus (MV) replication. Virus uptake into the target cells was not grossly altered by the two inhibitors, while titers of newly synthesized MV were reduced by approximately 1 log (90%) in PBL and 70-80% in BJAB cells. Lipidomic analyses revealed that in PBL SKI-II led to increased ceramide levels, whereas in BJAB cells ceranib-2 increased ceramides. SKI-II treatment decreased sphingosine-1-phosphate (S1P) levels in PBL and BJAB cells. Furthermore, we found that MV infection of lymphocytes induced a transient (0.5-6 h) increase in S1P, which was prevented by SKI-II. Investigating the effect of the inhibitors on the metabolic (mTORC1) activity we found that ceranib-2 reduced the phosphorylation of p70 S6K in PBL, and that both inhibitors, ceranib-2 and SKI-II, reduced the phosphorylation of p70 S6K in BJAB cells. As mTORC1 activity is required for efficient MV replication, this effect of the inhibitors is one possible antiviral mechanism. In addition, reduced intracellular S1P levels affect a number of signaling pathways and functions including Hsp90 activity, which was reported to be required for MV replication. Accordingly, we found that pharmacological inhibition of Hsp90 with the inhibitor 17-AAG strongly impaired MV replication in primary PBL. Thus, our data suggest that treatment of lymphocytes with both, acid ceramidase and SphK inhibitors, impair MV replication by affecting a number of cellular activities including mTORC1 and Hsp90, which alter the metabolic state of the cells causing a hostile environment for the virus.

Sphingolipids in viral infection

Viruses exploit membranes and their components such as sphingolipids in all steps of their life cycle including attachment and membrane fusion, intracellular transport, replication, protein sorting and budding. Examples for sphingolipid-dependent virus entry are found for: human immunodeficiency virus (HIV), which besides its protein receptors also interacts with glycosphingolipids (GSLs); rhinovirus, which promotes the formation of ceramide-enriched platforms and endocytosis; or measles virus (MV), which induces the surface expression of its own receptor CD150 via activation of sphingomyelinases (SMases). While SMase activation was implicated in Ebola virus (EBOV) attachment, the virus utilizes the cholesterol transporter Niemann-Pick C protein 1 (NPC1) as 'intracellular' entry receptor after uptake into endosomes. Differential activities of SMases also affect the intracellular milieu required for virus replication. Sindbis virus (SINV), for example, replicates better in cells lacking acid SMase (ASMase). Defined lipid compositions of viral assembly and budding sites influence virus release and infectivity, as found for hepatitis C virus (HCV) or HIV. And finally, viruses manipulate cellular signaling and the sphingolipid metabolism to their advantage, as for example influenza A virus (IAV), which activates sphingosine kinase 1 and the transcription factor NF-κB.

Host sphingomyelin increases West Nile virus infection in vivo

Flaviviruses, such as the dengue virus and the West Nile virus (WNV), are arthropod-borne viruses that represent a global health problem. The flavivirus lifecycle is intimately connected to cellular lipids. Among the lipids co-opted by flaviviruses, we have focused on SM, an important component of cellular membranes particularly enriched in the nervous system. After infection with the neurotropic WNV, mice deficient in acid sphingomyelinase (ASM), which accumulate high levels of SM in their tissues, displayed exacerbated infection. In addition, WNV multiplication was enhanced in cells from human patients with Niemann-Pick type A, a disease caused by a deficiency of ASM activity resulting in SM accumulation. Furthermore, the addition of SM to cultured cells also increased WNV infection, whereas treatment with pharmacological inhibitors of SM synthesis reduced WNV infection. Confocal microscopy analyses confirmed the association of SM with viral replication sites within infected cells. Our results unveil that SM metabolism regulates flavivirus infection in vivo and propose SM as a suitable target for antiviral design against WNV.

Viral infections and sphingolipids

Besides their essential role in the immune system, sphingolipids and their metabolites are potential key regulators in the life cycle of obligatory intracellular pathogens such as viruses. They are involved in lateral and vertical segregation of receptors required for attachment, membrane fusion and endocytosis, as well as in the intracellular replication, assembly and release of viruses. Glycosphingolipids may themselves act as receptors for viruses, such as Galactosylceramide for human immunodeficiency virus (HIV). In addition, sphingolipids and their metabolites are inseparably interwoven in signal transduction processes, dynamic alterations of the cytoskeleton, and the regulation of innate and intrinsic responses of infected target cells. Depending on the nature of the intracellular pathogen, they may support or inhibit infections. Understanding of the underlying mechanisms depending on the specific virus, immune control, and type of disease may open new avenues for therapeutic interventions.

Hyperresponsiveness of mice deficient in plasma-secreted sphingomyelinase reveals its pivotal role in early phase of host response

Plasma secretion of acid sphingomyelinase is a hallmark of cellular stress response resulting in the formation of membrane embedded ceramide-enriched lipid rafts and the reorganization of receptor complexes. Consistently, decompartmentalization of ceramide formation from inert sphingomyelin has been associated with signaling events and regulation of the cellular phenotype. Herein, we addressed the question of whether the secretion of acid sphingomyelinase is involved in host response during sepsis. We found an exaggerated clinical course in mice genetically deficient in acid sphingomyelinase characterized by an increased bacterial burden, an increased phagocytotic activity, and a more pronounced cytokine storm. Moreover, on a functional level, leukocyte-endothelial interaction was found diminished in sphingomyelinase-deficient animals corresponding to a distinct leukocytes’ phenotype with respect to rolling and sticking as well as expression of cellular surface proteins. We conclude that hydrolysis of membrane-embedded sphingomyelin, triggered by circulating sphingomyelinase, plays a pivotal role in the first line of defense against invading microorganisms. This function might be essential during the early phase of infection leading to an adaptive response of remote cells and tissues.

Using ASMase knockout mice to model human diseases

Acid sphingomyelinase (ASMase) is a key initiator of sphingomyelin/ceramide signal transduction activated by many stress stimuli. Over the past two decades, much progress has been made in defining the clinical relevance of sphingomyelin/ceramide signaling in numerous diseases using ASMase knockout mice. Organs that operate this pathway are numerous and the disease states regulated are diverse, with ceramide generation governing injury in tumor, gut, ovary, brain, lung, heart, liver, and during infection. This chapter emphasizes evolutionary conservation of sphingolipid stress signaling and mammalian adaptations that permit transduction of organotypic responses. Recognition that the sphingomyelin/ceramide transducer calibrates extent of tissue injury, ultimately acting as a molecular switch that determines organ fate, is driving development of new pharmacologic concepts and tools to intervene therapeutically.

Brain pathology in Niemann Pick disease type A: insights from the acid sphingomyelinase knockout mice

Severe neurological involvement characterizes Niemann Pick disease (NPD) type A, an inherited disorder caused by loss of function mutations in the gene encoding acid sphingomyelinase (ASM). Mice lacking ASM, which mimic NPD type A, have provided important insights into the aberrant brain phenotypes induced by ASM deficiency. For example, lipid alterations, including the accumulation of sphingolipids, affect the membranes of different subcellular compartments of neurons and glial cells, leading to anomalies in signalling pathways, neuronal polarization, calcium homeostasis, synaptic plasticity, myelin production or immune response. These findings contribute to our understanding of the overall role of sphingolipids and their metabolic enzymes in brain physiology, and pave the way to design and test new therapeutic strategies for type A NPD and other neurodegenerative disorders. Some of these have already been tested in mice lacking ASM with promising results.

Ceramide in stress response

Evidence has consistently indicated that activation of sphingomyelinases and/or ceramide synthases and the resulting accumulation of ceramide mediate cellular responses to stressors such as lipopolysaccharide, interleukin 1beta, tumor necrosis factor alpha, serum deprivation, irradiation and various antitumor treatments. Recent studies had identified the genes encoding most of the enzymes responsible for the generation of ceramide and ongoing research is aimed at characterizing their individual functions in cellular response to stress. This chapter discusses the seminal and more recent discoveries in regards to the pathways responsible for the accumulation of ceramide during stress and the mechanisms by which ceramide affects cell functions. The former group includes the roles of neutral sphingomyelinase 2, serine palmitoyltransferase, ceramide synthases, as well as the secretory and endosomal/lysosomal forms of acid sphingomyelinase. The latter summarizes the mechanisms by which ceramide activate its direct targets, PKCzeta, PP2A and cathepsin D. The ability of ceramide to affect membrane organization is discussed in the light of its relevance to cell signaling. Emerging evidence to support the previously assumed notion that ceramide acts in a strictly structure-specific manner are also included. These findings are described in the context of several physiological and pathophysiological conditions, namely septic shock, obesity-induced insulin resistance, aging and apoptosis of tumor cells in response to radiation and chemotherapy.

Syntaxin 4 is required for acid sphingomyelinase activity and apoptotic function

Acid sphingomyelinase (A-SMase) is an important enzyme in sphingolipid metabolism and plays key roles in apoptosis, immunity, development, and cancer. In addition, it mediates cytotoxicity of cisplatin and some other chemotherapeutic drugs. The mechanism of A-SMase activation is still undefined. We now demonstrate that, upon CD95 stimulation, A-SMase is activated through translocation from intracellular compartments to the plasma membrane in an exocytic pathway requiring the t-SNARE protein syntaxin 4. Indeed, down-regulation of syntaxin 4 inhibits A-SMase translocation and activation induced by CD95 stimulation. This leads to inhibition of the CD95-triggered signaling events, including caspase 3 and 9 activation and apoptosis, activation of the survival pathway involving the protein kinase Akt, and important changes in cell cycle and proliferation. The molecular interaction between A-SMase and syntaxin 4 was not known and clarifies the mechanism of A-SMase activation. The novel actions of syntaxin 4 in sphingolipid metabolism and exocytosis we describe here define signaling mechanisms of broad relevance in cell pathophysiology.

Induction of membrane ceramides: a novel strategy to interfere with T lymphocyte cytoskeletal reorganisation in viral immunosuppression

Silencing of T cell activation and function is a highly efficient strategy of immunosuppression induced by pathogens. By promoting formation of membrane microdomains essential for clustering of receptors and signalling platforms in the plasma membrane, ceramides accumulating as a result of membrane sphingomyelin breakdown are not only essential for assembly of signalling complexes and pathogen entry, but also act as signalling modulators, e. g. by regulating relay of phosphatidyl-inositol-3-kinase (PI3K) signalling. Their role in T lymphocyte functions has not been addressed as yet. We now show that measles virus (MV), which interacts with the surface of T cells and thereby efficiently interferes with stimulated dynamic reorganisation of their actin cytoskeleton, causes ceramide accumulation in human T cells in a neutral (NSM) and acid (ASM) sphingomyelinase-dependent manner. Ceramides induced by MV, but also bacterial sphingomyelinase, efficiently interfered with formation of membrane protrusions and T cell spreading and front/rear polarisation in response to beta1 integrin ligation or alphaCD3/CD28 activation, and this was rescued upon pharmacological or genetic ablation of ASM/NSM activity. Moreover, membrane ceramide accumulation downmodulated chemokine-induced T cell motility on fibronectin. Altogether, these findings highlight an as yet unrecognised concept of pathogens able to cause membrane ceramide accumulation to target essential processes in T cell activation and function by preventing stimulated actin cytoskeletal dynamics.

Acid sphingomyelinase activity triggers microparticle release from glial cells

We have earlier shown that microglia, the immune cells of the CNS, release microparticles from cell plasma membrane after ATP stimulation. These vesicles contain and release IL-1β, a crucial cytokine in CNS inflammatory events. In this study, we show that microparticles are also released by astrocytes and we get insights into the mechanism of their shedding. We show that, on activation of the ATP receptor P2X₇, microparticle shedding is associated with rapid activation of acid sphingomyelinase, which moves to plasma membrane outer leaflet. ATP-induced shedding and IL-1β release are markedly reduced by the inhibition of acid sphingomyelinase, and completely blocked in glial cultures from acid sphingomyelinase knockout mice. We also show that p38 MAPK cascade is relevant for the whole process, as specific kinase inhibitors strongly reduce acid sphingomyelinase activation, microparticle shedding and IL-1β release. Our results represent the first demonstration that activation of acid sphingomyelinase is necessary and sufficient for microparticle release from glial cells and define key molecular effectors of microparticle formation and IL-1β release, thus, opening new strategies for the treatment of neuroinflammatory diseases.

Effect of host cell lipid metabolism on alphavirus replication, virion morphogenesis, and infectivity

The alphavirus Sindbis virus (SINV) causes encephalomyelitis in mice. Lipid-containing membranes, particularly cholesterol and sphingomyelin (SM), play important roles in virus entry, RNA replication, glycoprotein transport, and budding. Levels of SM are regulated by sphingomyelinases (SMases). Acid SMase (ASMase) deficiency results in the lipid storage disease type A Niemann-Pick disease (NPD-A), mimicked in mice by interruption of the ASMase gene. We previously demonstrated that ASMase-deficient mice are more susceptible to fatal SINV encephalomyelitis, with increased viral replication, spread, and neuronal death. To determine the mechanisms by which ASMase deficiency enhances SINV replication, we compared NPD-A fibroblasts (NPAF) to normal human fibroblasts (NHF). NPAF accumulated cholesterol- and sphingolipid-rich late endosomes/lysosomes in the perinuclear region. SINV replication was faster and reached higher titer in NPAF than in NHF, and NPAF died more quickly. SINV RNA and protein synthesis was greater in NHF than in NPAF, but virions budding from NPAF were 26 times more infectious and were regular dense particles whereas virions from NHF were larger particles containing substantial amounts of CD63. Cellular regulation of alphavirus morphogenesis is a previously unrecognized mechanism for control of virus replication and spread.

The unexpected role of acid sphingomyelinase in cell death and the pathophysiology of common diseases

Acid sphingomyelinase (ASM; E.C. 3.1.4.12) is best known for its involvement in the lysosomal storage disorder Niemann-Pick disease (NPD). Through studies that began by investigating this rare disease, recent findings have uncovered the important role of this enzyme in the initiation of ceramide-mediated signal transduction. This unique function involves translocation of the enzyme from intracellular compartments to the outer leaflet of the cell membrane, where hydrolysis of sphingomyelin into ceramide initiates membrane reorganization and facilitates the formation and coalescence of lipid microdomains. These microdomains are sites of protein-protein interactions that lead to downstream signaling, and perturbation of microdomain formation influences the pathophysiology of many common diseases. The initial observations implicating ASM in this process have come from studies using cells from patients with NPD or from ASM knockout (ASMKO) mice, where the genetic deficiency of this enzymatic activity has been shown to protect these cells and animals from stress-induced and developmental apoptosis. This review will discuss the complex biology of this enzyme in the context of these new findings and its recently reported importance in common human diseases, including cancer, sepsis, cardiovascular, pulmonary, liver, and neurological diseases as well as the potential for using ASM (or ASM inhibitors) as therapeutic agents.

Functions of sphingolipid metabolism in mammals--lessons from genetic defects

Much is known about the pathways that control the biosynthesis, transport and degradation of sphingolipids. During the last two decades, considerable progress has been made regarding the roles this complex group of lipids play in maintaining membrane integrity and modulating responses to numerous signals. Further novel insights have been provided by the analysis of newly discovered genetic diseases in humans as well as in animal models harboring mutations in the genes whose products control sphingolipid metabolism and action. Through the description of the phenotypic consequences of genetic defects resulting in the loss of activity of the many proteins that synthesize, transport, bind, or degrade sphingolipids, this review summarizes the (patho)physiological functions of these lipids.

Sphingolipid metabolizing enzymes as novel therapeutic targets

Pharmacological interference with sphingolipid metabolizing enzymes promises to provide novel ways to modulate cellular pathways relevant in multiple diseases. In this review, we focus on two sphingolipid signaling molecules, sphingosine-1-phosphate (S1P) and ceramide, as they are involved in cell fate decisions (survival vs. apoptosis) and in a wide range of pathophysiological processes. For S1P, we will discuss sphingosine kinases and S1P lyase as the enzymes which are crucial for its production and degradation, respectively, emphasizing the potential therapeutic usefulness of inhibitors of these enzymes. For ceramide, we will concentrate on acid sphingomyelinase, and critically review the substantial literature which implicates this enzyme as a worthwhile target for pharmacological inhibitors. It will become clear that the task to validate these enzymes as drug targets is not finished and many questions regarding the therapeutic usefulness of their inhibitors remain unanswered. Still this approach holds promise for a number of totally new therapies, and, on the way, detailed insight into sphingolipid signaling pathways can be gained.