Ceramidase regulates synaptic vesicle exocytosis and trafficking

Secretomic changes of amyloid beta peptides on Alzheimer's disease related proteins in differentiated human SH-SY5Y neuroblastoma cells

Alzheimer's disease (AD) is a neurodegenerative disease that causes physical damage to neuronal connections, leading to brain atrophy. This disruption of synaptic connections results in mild to severe cognitive impairments. Unfortunately, no effective treatment is currently known to prevent or reverse the symptoms of AD. The aim of this study was to investigate the effects of three synthetic peptides, i.e., KLVFF, RGKLVFFGR and RIIGL, on an AD in vitro model represented by differentiated SH-SY5Y neuroblastoma cells exposed to retinoic acid (RA) and brain-derived neurotrophic factor (BDNF). The results demonstrated that RIIGL peptide had the least significant cytotoxic activity to normal SH-SY5Y while exerting high cytotoxicity against the differentiated cells. The mechanism of RIIGL peptide in the differentiated SH-SY5Y was investigated based on changes in secretory proteins compared to another two peptides. A total of 380 proteins were identified, and five of them were significantly detected after treatment with RIIGL peptide. These secretory proteins were found to be related to microtubule-associated protein tau (MAPT) and amyloid-beta precursor protein (APP). RIIGL peptide acts on differentiated SH-SY5Y by regulating amyloid-beta formation, neuron apoptotic process, ceramide catabolic process, and oxidative phosphorylation and thus has the potentials to treat AD.

Methodological Pitfalls of Investigating Lipid Rafts in the Brain: What Are We Still Missing?

The purpose of this review is to succinctly examine the methodologies used in lipid raft research in the brain and to highlight the drawbacks of some investigative approaches. Lipid rafts are biochemically and biophysically different from the bulk membrane. A specific lipid environment within membrane domains provides a harbor for distinct raftophilic proteins, all of which in concert create a specialized platform orchestrating various cellular processes. Studying lipid rafts has proved to be arduous due to their elusive nature, mobility, and constant dynamic reorganization to meet the cellular needs. Studying neuronal lipid rafts is particularly cumbersome due to the immensely complex regional molecular architecture of the central nervous system. Biochemical fractionation, performed with or without detergents, is still the most widely used method to isolate lipid rafts. However, the differences in solubilization when various detergents are used has exposed a dire need to find more reliable methods to study particular rafts. Biochemical methods need to be complemented with other approaches such as live-cell microscopy, imaging mass spectrometry, and the development of specific non-invasive fluorescent probes to obtain a more complete image of raft dynamics and to study the spatio-temporal expression of rafts in live cells.

Lipid-dependent regulation of neurotransmitter release from sympathetic nerve endings in mice atria

Neurotransmitter release from sympathetic terminals is a key avenue for heart regulation. Herein, presynaptic exocytotic activity was monitored in mice atrial tissue using a false fluorescent neurotransmitter FFN511, a substrate for monoamine transporters. FFN511 labeling had similarity with tyrosine hydroxylase immunostaining. High [K+]o depolarization caused FFN511 release, which was augmented by reserpine, an inhibitor of neurotransmitter uptake. However, reserpine lost the ability to increase depolarization-induced FFN511 unloading after depletion of ready releasable pool with hyperosmotic sucrose. Cholesterol oxidase and sphingomyelinase modified atrial membranes, changing in opposite manner fluorescence of lipid ordering-sensitive probe. Plasmalemmal cholesterol oxidation increased FFN511 release upon K+-depolarization and more markedly potentiated FFN511 unloading in the presence of reserpine. Hydrolysis of plasmalemmal sphingomyelin profoundly enhanced the rate of FFN511 loss due to K+-depolarization, but completely prevented potentiating action of reserpine on FFN511 unloading. If cholesterol oxidase or sphingomyelinase got access to membranes of recycling synaptic vesicles, then the enzyme effects were suppressed. Hence, a fast neurotransmitter reuptake dependent on exocytosis of vesicles from ready releasable pool occurs during presynaptic activity. This reuptake can be enhanced or inhibited by plasmalemmal cholesterol oxidation or sphingomyelin hydrolysis, respectively. These modifications of plasmalemmal (but not vesicular) lipids increase the evoked neurotransmitter release.

The role of lipid rafts in vesicle formation

The formation of membrane vesicles is a common feature in all eukaryotes. Lipid rafts are the best-studied example of membrane domains for both eukaryotes and prokaryotes, and their existence also is suggested in Archaea membranes. Lipid rafts are involved in the formation of transport vesicles, endocytic vesicles, exocytic vesicles, synaptic vesicles and extracellular vesicles, as well as enveloped viruses. Two mechanisms of how rafts are involved in vesicle formation have been proposed: first, that raft proteins and/or lipids located in lipid rafts associate with coat proteins that form a budding vesicle, and second, vesicle budding is triggered by enzymatic generation of cone-shaped ceramides and inverted cone-shaped lyso-phospholipids. In both cases, induction of curvature is also facilitated by the relaxation of tension in the raft domain. In this Review, we discuss the role of raft-derived vesicles in several intracellular trafficking pathways. We also highlight their role in different pathways of endocytosis, and in the formation of intraluminal vesicles (ILVs) through budding inwards from the multivesicular body (MVB) membrane, because rafts inside MVB membranes are likely to be involved in loading RNA into ILVs. Finally, we discuss the association of glycoproteins with rafts via the glycocalyx.

Sphingomyelinase modulates synaptic vesicle mobilization at the mice neuromuscular junctions

AIMS

Sphingomyelin is an abundant component of the presynaptic membrane and an organizer of lipid rafts. In several pathological conditions, sphingomyelin is hydrolyzed due to an upregulation and release of secretory sphingomyelinases (SMases). Herein, the effects of SMase on exocytotic neurotransmitter release were studied in the diaphragm neuromuscular junctions of mice.

MAIN METHODS

Microelectrode recordings of postsynaptic potentials and styryl (FM) dyes were used to estimate neuromuscular transmission. Membrane properties were assessed with fluorescent techniques.

KEY FINDINGS

Application of SMase at a low concentration (0.01 U ml^-1) led to a disruption of lipid-packing in the synaptic membranes. Neither spontaneous exocytosis nor evoked neurotransmitter release (in response to single stimuli) were affected by SMase treatment. However, SMase significantly increased neurotransmitter release and the rate of fluorescent FM-dye loss from the synaptic vesicles at 10, 20 and 70 Hz stimulation of the motor nerve. In addition, SMase treatment prevented a shift of the exocytotic mode from "full-collapse" fusion to "kiss-and-run" during high-frequency (70 Hz) activity. The potentiating effects of SMase on neurotransmitter release and FM-dye unloading were suppressed when synaptic vesicle membranes were also exposed to this enzyme (i.e., stimulation occurred during SMase treatment).

SIGNIFICANCE

Thus, hydrolysis of the plasma membrane sphingomyelin can enhance mobilization of synaptic vesicles and facilitate full fusion mode of exocytosis, but SMase acting on vesicular membrane had a depressant effect on the neurotransmission. Partially, the effects of SMase can be related with the changes in synaptic membrane properties and intracellular signaling.

Lipids and Secretory Vesicle Exocytosis

In recent years, the number of studies implicating lipids in the regulation of synaptic vesicle exocytosis has risen considerably. It has become increasingly clear that lipids such as phosphoinositides, lysophospholipids, cholesterol, arachidonic acid and myristic acid play critical regulatory roles in the processes leading up to exocytosis. Lipids may affect membrane fusion reactions by altering the physical properties of the membrane, recruiting key regulatory proteins, concentrating proteins into exocytic "hotspots" or by modulating protein functions allosterically. Discrete changes in phosphoinositides concentration are involved in multiple trafficking events including exocytosis and endocytosis. Lipid-modifying enzymes such as the DDHD2 isoform of phospholipase A1 were recently shown to contribute to memory acquisition via dynamic modifications of the brain lipid landscape. Considering the increasing reports on neurodegenerative disorders associated with aberrant intracellular trafficking, an improved understanding of the control of lipid pathways is physiologically and clinically significant and will afford unique insights into mechanisms and therapeutic methods for neurodegenerative diseases. Consequently, this chapter will discuss the different classes of lipids, phospholipase enzymes, the evidence linking them to synaptic neurotransmitter release and how they act to regulate key steps in the multi-step process leading to neuronal communication and memory acquisition.

Sphingosine kinase 2 regulates insulin receptor trafficking in hepatocytes

Disturbed insulin receptor (InsR) trafficking is associated with impaired insulin signaling and the development of diabetes. Sphingosine kinase (SphK), including SphK1 and SphK2, is a key enzyme of sphingolipid metabolism, which has been implicated in the regulation of membrane trafficking. More recently, we have reported that SphK2 is a key regulator of hepatic insulin signaling and glucose homeostasis. However, the role of SphK in InsR trafficking is still undefined. Huh7 cells were treated with specific SphK1 and SphK2 inhibitors or SphK1- and SphK2-specific small interfering RNA (siRNA) in the presence or absence of insulin. Flow cytometry and immunofluorescence assays were carried out to investigate the role of SphK in InsR trafficking. InsR endocytosis, recycling, and insulin signaling were analyzed. Inhibition of SphK2, but not SphK1, by either specific pharmaceutic inhibitors or siRNA, significantly suppressed InsR endocytosis and recycling following insulin stimulation. Consequently, the insulin-stimulated Akt activation was significantly attenuated by SphK2 inhibition in hepatocytes. Moreover, the effect of SphK2 on InsR trafficking was mediated via the clathrin-dependent mechanism. Thus, our results show that SphK2 is able to regulate InsR trafficking. These findings suggest that SphK2 may impinge on hepatic insulin signaling by regulating InsR trafficking, providing further mechanistic evidence that SphK2 could serve as a potential intervention target against insulin resistance and T2D (type 2 diabetes).

Extracellular vesicles, from the pathogenesis to the therapy of neurodegenerative diseases

Extracellular vesicles (EVs) are small bilipid layer-enclosed vesicles that can be secreted by all tested types of brain cells. Being a key intercellular communicator, EVs have emerged as a key contributor to the pathogenesis of various neurodegenerative diseases (NDs) including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease through delivery of bioactive cargos within the central nervous system (CNS). Importantly, CNS cell-derived EVs can be purified via immunoprecipitation, and EV cargos with altered levels have been identified as potential biomarkers for the diagnosis and prognosis of NDs. Given the essential impact of EVs on the pathogenesis of NDs, pathological EVs have been considered as therapeutic targets and EVs with therapeutic effects have been utilized as potential therapeutic agents or drug delivery platforms for the treatment of NDs. In this review, we focus on recent research progress on the pathological roles of EVs released from CNS cells in the pathogenesis of NDs, summarize findings that identify CNS-derived EV cargos as potential biomarkers to diagnose NDs, and comprehensively discuss promising potential of EVs as therapeutic targets, agents, and drug delivery systems in treating NDs, together with current concerns and challenges for basic research and clinical applications of EVs regarding NDs.

Proline Metabolism in Neurological and Psychiatric Disorders

Proline plays a multifaceted role in protein synthesis, redox balance, cell fate regulation, brain development, and other cellular and physiological processes. Here, we focus our review on proline metabolism in neurons, highlighting the role of dysregulated proline metabolism in neuronal dysfunction and consequently neurological and psychiatric disorders. We will discuss the association between genetic and protein function of enzymes in the proline pathway and the development of neurological and psychiatric disorders. We will conclude with a potential mechanism of proline metabolism in neuronal function and mental health.

Neuronal activity induces glucosylceramide that is secreted via exosomes for lysosomal degradation in glia

Recessive variants in GBA1 cause Gaucher disease, a prevalent form of lysosome storage disease. GBA1 encodes a lysosomal enzyme that hydrolyzes glucosylceramide (GlcCer) into glucose and ceramide. Its loss causes lysosomal dysfunction and increased levels of GlcCer. We generated a null allele of the Drosophila ortholog Gba1b by inserting the Gal4 using CRISPR-Cas9. Here, we show that Gba1b is expressed in glia but not in neurons. Glial-specific knockdown recapitulates the defects found in Gba1b mutants, and these can be rescued by glial expression of human GBA1. We show that GlcCer is synthesized upon neuronal activity, and it is transported from neurons to glia through exosomes. Furthermore, we found that glial TGF-β/BMP induces the transfer of GlcCer from neurons to glia and that the White protein, an ABCG transporter, promotes GlcCer trafficking to glial lysosomes for degradation.

RNA-binding FMRP and Staufen sequentially regulate the Coracle scaffold to control synaptic glutamate receptor and bouton development

Both mRNA-binding Fragile X mental retardation protein (FMRP; Fmr1) and mRNA-binding Staufen regulate synaptic bouton formation and glutamate receptor (GluR) levels at the Drosophila neuromuscular junction (NMJ) glutamatergic synapse. Here, we tested whether these RNA-binding proteins act jointly in a common mechanism. We found that both dfmr1 and staufen mutants, and trans-heterozygous double mutants, displayed increased synaptic bouton formation and GluRIIA accumulation. With cell-targeted RNA interference, we showed a downstream Staufen role within postsynaptic muscle. With immunoprecipitation, we showed that FMRP binds staufen mRNA to stabilize postsynaptic transcripts. Staufen is known to target actin-binding, GluRIIA anchor Coracle, and we confirmed that Staufen binds to coracle mRNA. We found that FMRP and Staufen act sequentially to co-regulate postsynaptic Coracle expression, and showed that Coracle, in turn, controls GluRIIA levels and synaptic bouton development. Consistently, we found that dfmr1, staufen and coracle mutants elevate neurotransmission strength. We also identified that FMRP, Staufen and Coracle all suppress pMad activation, providing a trans-synaptic signaling linkage between postsynaptic GluRIIA levels and presynaptic bouton development. This work supports an FMRP-Staufen-Coracle-GluRIIA-pMad pathway regulating structural and functional synapse development.

An overview of the synaptic vesicle lipid composition

Chemical neurotransmission is the major mechanism of neuronal communication. Neurotransmitters are released from secretory organelles, the synaptic vesicles (SVs) via exocytosis into the synaptic cleft. Fusion of SVs with the presynaptic plasma membrane is balanced by endocytosis, thus maintaining the presynaptic membrane at steady-state levels. The protein machineries responsible for exo- and endocytosis have been extensively investigated. In contrast, less is known about the role of lipids in synaptic transmission and how the lipid composition of SVs is affected by dynamic exo-endocytotic cycling. Here we summarize the current knowledge about the composition, organization, and function of SV membrane lipids. We also cover lipid biogenesis and maintenance during the synaptic vesicle cycle.

Perinatal Dietary Polyunsaturated Fatty Acids in Brain Development, Role in Neurodevelopmental Disorders

n-3 and n-6 polyunsaturated fatty acids (PUFAs) are essential fatty acids that are provided by dietary intake. Growing evidence suggests that n-3 and n-6 PUFAs are paramount for brain functions. They constitute crucial elements of cellular membranes, especially in the brain. They are the precursors of several metabolites with different effects on inflammation and neuron outgrowth. Overall, long-chain PUFAs accumulate in the offspring brain during the embryonic and post-natal periods. In this review, we discuss how they accumulate in the developing brain, considering the maternal dietary supply, the polymorphisms of genes involved in their metabolism, and the differences linked to gender. We also report the mechanisms linking their bioavailability in the developing brain, their transfer from the mother to the embryo through the placenta, and their role in brain development. In addition, data on the potential role of altered bioavailability of long-chain n-3 PUFAs in the etiologies of neurodevelopmental diseases, such as autism, attention deficit and hyperactivity disorder, and schizophrenia, are reviewed.

Early differences in membrane properties at the neuromuscular junctions of ALS model mice: Effects of 25-hydroxycholesterol

AIMS

Plasma hyperlipidemia is a protective factor in amyotrophic lateral sclerosis (ALS) while cholesterol-lowering drugs aggravate the pathology. We hypothesize that this phenomenon can be linked with membrane lipid alterations in the neuromuscular junctions (NMJs) occurring before motor neuron loss.

METHODS

Neurotransmitter release in parallel with lipid membrane properties in diaphragm NMJs of SOD1G93A (mSOD) mice at nine weeks of age (pre-onset stage) were assessed.

KEY FINDINGS

Despite on slight changes in spontaneous and evoked quantum release of acetylcholine, extracellular levels of choline at resting conditions, an indicator of non-quantum release, were significantly increased in mSOD mice. The use of lipid-sensitive fluorescent probes points to lipid raft disruption in the NMJs of mSOD mice. However, content of cholesterol, a key raft component was unchanged implying another pathway responsible for the loss of raft integrity. In the mSOD mice we found marked increase in levels of raft-destabilizing lipid ceramide. This was accompanied by enhanced ability to uptake of exogenous ceramide in NMJs. Acute and chronic administration of 25-hydroxycholesterol, whose levels increase due to hypercholesterolemia, recovered early alterations in membrane properties. Furthermore, chronic treatment with 25-hydroxycholesterol prevented increase in ceramide and extracellular choline levels as well as suppressed lipid peroxidation of NMJ membranes and fragmentation of end plates.

SIGNIFICANCE

Thus, lipid raft disruption likely due to ceramide accumulation could be early event in ALS which may trigger neuromuscular abnormalities. Cholesterol derivative 25-hydroxycholesterol may serve as a molecule restoring the membrane and functional properties of NMJs at the early stage.

Interaction of Synaptosomal-Associated Protein 25 with Neutral Sphingomyelinase 2: Functional Impact on the Sphingomyelin Pathway

Neurotransmitter release is mediated by ceramide, which is generated by sphingomyelin hydrolysis. In the present study, we examined whether synaptosomal-associated protein 25 (SNAP-25) is involved in ceramide production and exocytosis. Neutral sphingomyelinase 2 (nSMase2) was partially purified from bovine brain and we found that SNAP-25 was enriched in the nSMase2-containing fractions. In rat synaptosomes and PC12 cells, the immunoprecipitation pellet of anti-SNAP-25 antibody showed higher nSMase activity than the immunoprecipitation pellet of anti-nSMase2 antibody. In PC12 cells, SNAP-25 was colocalized with nSMase2. Transfection of SNAP-25 small interfering RNA (siRNA) significantly inhibited nSMase2 translocation to the plasma membrane. A23187-induced ceramide production was concomitantly reduced in SNAP-25 siRNA-transfected PC12 cells compared with that in scrambled siRNA-transfected cells. Moreover, transfection of SNAP-25 siRNA inhibited dopamine release, whereas addition of C₆-ceramide to the siRNA-treated cells moderately reversed this inhibition. Additionally, nSMase2 inhibition reduced dopamine release. Collectively, our results indicate that SNAP-25 interacts with nSMase2 during ceramide production, which mediates exocytosis and neurotransmitter release.

Sphingolipids in Alzheimer's disease, how can we target them?

Altered levels of sphingolipids and their metabolites in the brain, and the related downstream effects on neuronal homeostasis and the immune system, provide a framework for understanding mechanisms in neurodegenerative disorders and for developing new intervention strategies. In this review we will discuss: the metabolites of sphingolipids that function as second messengers; and functional aberrations of the pathway resulting in Alzheimer's disease (AD) pathophysiology. Focusing on the central product of the sphingolipid pathway ceramide, we describ approaches to pharmacologically decrease ceramide levels in the brain and we argue on how the sphingolipid pathway may represent a new framework for developing novel intervention strategies in AD. We also highlight the possible use of clinical and non-clinical drugs to modulate the sphingolipid pathway and sphingolipid-related biological cascades.

Changes in Membrane Ceramide Pools in Rat Soleus Muscle in Response to Short-Term Disuse

Lipid raft disruption is an early event during skeletal muscle unloading. Ceramide (Cer) serves as a signaling lipid that can contribute to lipid raft disturbance and muscle atrophy. Using biochemical and fluorescent approaches, the distribution of Cer and related molecules in the rat soleus muscle subjected to 12 h of hindlimb suspension (HS) was studied. HS led to upregulation of TNFα receptor 1 (TNFR1), Cer-producing enzymes, and acid and neutral sphingomyelinase (SMase) in detergent-resistant membranes (lipid rafts), which was accompanied by an increase in Cer and a decrease in sphingomyelin in this membrane fraction. Fluorescent labeling indicated increased Cer in the sarcoplasm as well as the junctional (synaptic) and extrajunctional compartments of the suspended muscles. Also, a loss of membrane asymmetry (a hallmark of membrane disturbance) was induced by HS. Pretreatment with clomipramine, a functional inhibitor of acid SMase, counteracted HS-mediated changes in the Cer/sphingomyelin ratio and acid SMase abundance as well as suppressed Cer accumulation in the intracellular membranes of junctional and extrajunctional regions. However, the elevation of plasma membrane Cer and disturbance of the membrane asymmetry were suppressed only in the junctional compartment. We suggest that acute HS leads to TNFR1 and SMase upregulation in the lipid raft fraction and deposition of Cer throughout the sarcolemma and intracellularly. Clomipramine-mediated downregulation of acid SMase can suppress Cer accumulation in all compartments, excluding the extrajunctional plasma membrane.

Novel Cellular Functions of Very Long Chain-Fatty Acids: Insight From ELOVL4 Mutations

Elongation of Very Long chain fatty acids-4 (ELOVL4) protein is a member of the ELOVL family of fatty acid elongases that is collectively responsible for catalyzing formation of long chain fatty acids. ELOVL4 is the only family member that catalyzes production of Very Long Chain Saturated Fatty Acids (VLC-SFA) and Very Long Chain Polyunsaturated Fatty Acids (VLC-PUFA) with chain lengths ≥28 carbons. ELOVL4 and its VLC-SFA and VLC-PUFA products are emerging as important regulators of synaptic signaling and neuronal survival in the central nervous system (CNS). Distinct sets of mutations in ELOVL4 cause three different neurological diseases in humans. Heterozygous inheritance of one set of autosomal dominant ELOVL4 mutations that leads to truncation of the ELOVL4 protein causes Stargardt-like macular dystrophy (STGD3), an aggressive juvenile-onset retinal degeneration. Heterozygous inheritance of a different set of autosomal dominant ELOVL4 mutations that leads to a full-length protein with single amino acid substitutions causes spinocerebellar ataxia 34 (SCA34), a late-onset neurodegenerative disease characterized by gait ataxia and cerebellar atrophy. Homozygous inheritance of a different set of ELOVL4 mutations causes a more severe disease with infantile onset characterized by seizures, spasticity, intellectual disability, ichthyosis, and premature death. ELOVL4 is expressed widely in the CNS and is found primarily in neurons. ELOVL4 is expressed in cell-specific patterns within different regions of the CNS that are likely to be related to disease symptoms. In the retina, ELOVL4 is expressed exclusively in photoreceptors and produces VLC-PUFA that are incorporated into phosphatidylcholine and enriched in the light sensitive membrane disks of the photoreceptor outer segments. VLC-PUFA are enzymatically converted into "elovanoid" compounds that appear to provide paracrine signals that promote photoreceptor and neuronal survival. In the brain, the main ELOVL4 products are VLC-SFA that are incorporated into sphingolipids and enriched in synaptic vesicles, where they regulate kinetics of presynaptic neurotransmitter release. Understanding the function of ELOVL4 and its VLC-SFA and VLC-PUFA products will advance our understanding of basic mechanisms in neural signaling and has potential for developing novel therapies for seizure and neurodegenerative diseases.

ELOVL4: Very long-chain fatty acids serve an eclectic role in mammalian health and function

ELOngation of Very Long chain fatty acids-4 (ELOVL4) is an elongase responsible for the biosynthesis of very long chain (VLC, ≥C28) saturated (VLC-SFA) and polyunsaturated (VLC-PUFA) fatty acids in brain, retina, skin, Meibomian glands, and testes. Fascinatingly, different mutations in this gene have been reported to cause vastly different phenotypes in humans. Heterozygous inheritance of seven different mutations in the coding sequence and 5' untranslated region of ELOVL4 causes autosomal dominant Stargardt-like macular dystrophy (STGD3), while homozygous inheritance of three more mutant variants causes severe seizures with ichthyosis, hypertonia, and even death. Some recent studies have described heterozygous inheritance in yet another three mutant ELOVL4 variants, two that cause spinocerebellar ataxia-34 (SCA34) with erythrokeratodermia (EKV) and one that causes SCA34 without EKV. We identified the specific enzymatic reactions catalyzed by ELOVL4 and, using a variety of genetically engineered mouse models, have actively searched for the mechanisms by which ELOVL4 impacts neural function and health. In this review, we critically compare and contrast the various animal model and case studies involving ELOVL4 deficiency via either mutation or deletion, and the resulting consequences on neuronal health and function in both the retina and central nervous system.

The 16p11.2 homologs fam57ba and doc2a generate certain brain and body phenotypes

Deletion of the 16p11.2 CNV affects 25 core genes and is associated with multiple symptoms affecting brain and body, including seizures, hyperactivity, macrocephaly, and obesity. Available data suggest that most symptoms are controlled by haploinsufficiency of two or more 16p11.2 genes. To identify interacting 16p11.2 genes, we used a pairwise partial loss of function antisense screen for embryonic brain morphology, using the accessible zebrafish model. fam57ba, encoding a ceramide synthase, was identified as interacting with the doc2a gene, encoding a calcium-sensitive exocytosis regulator, a genetic interaction not previously described. Using genetic mutants, we demonstrated that doc2a^+/− fam57ba^+/− double heterozygotes show hyperactivity and increased seizure susceptibility relative to wild-type or single doc2a^−/− or fam57ba^−/− mutants. Additionally, doc2a^+/− fam57ba^+/− double heterozygotes demonstrate the increased body length and head size. Single doc2a^+/− and fam57ba^+/− heterozygotes do not show a body size increase; however, fam57ba^−/− homozygous mutants show a strongly increased head size and body length, suggesting a greater contribution from fam57ba to the haploinsufficient interaction between doc2a and fam57ba. The doc2a^+/− fam57ba^+/− interaction has not been reported before, nor has any 16p11.2 gene previously been linked to increased body size. These findings demonstrate that one pair of 16p11.2 homologs can regulate both brain and body phenotypes that are reflective of those in people with 16p11.2 deletion. Together, these findings suggest that dysregulation of ceramide pathways and calcium sensitive exocytosis underlies seizures and large body size associated with 16p11.2 homologs in zebrafish. The data inform consideration of mechanisms underlying human 16p11.2 deletion symptoms.

Homozygous Expression of Mutant ELOVL4 Leads to Seizures and Death in a Novel Animal Model of Very Long-Chain Fatty Acid Deficiency

Lipids are essential components of the nervous system. However, the functions of very long-chain fatty acids (VLC-FA; ≥ 28 carbons) in the brain are unknown. The enzyme ELOngation of Very Long-chain fatty acids-4 (ELOVL4) catalyzes the rate-limiting step in the biosynthesis of VLC-FA (Agbaga et al., Proc Natl Acad Sci USA 105(35): 12843-12848, 2008; Logan et al., J Lipid Res 55(4): 698-708, 2014), which we identified in the brain as saturated fatty acids (VLC-SFA). Homozygous mutations in ELOVL4 cause severe neuropathology in humans (Ozaki et al., JAMA Neurol 72(7): 797-805, 2015; Mir et al., BMC Med Genet 15: 25, 2014; Cadieux-Dion et al., JAMA Neurol 71(4): 470-475, 2014; Bourassa et al., JAMA Neurol 72(8): 942-943, 2015; Aldahmesh et al., Am J Hum Genet 89(6): 745-750, 2011) and are post-natal lethal in mice (Cameron et al., Int J Biol Sci 3(2): 111-119, 2007; Li et al., Int J Biol Sci 3(2): 120-128, 2007; McMahon et al., Molecular Vision 13: 258-272, 2007; Vasireddy et al., Hum Mol Genet 16(5): 471-482, 2007) from dehydration due to loss of VLC-SFA that comprise the skin permeability barrier. Double transgenic mice with homozygous knock-in of the Stargardt-like macular dystrophy (STDG3; 797-801_AACTT) mutation of Elovl4 with skin-specific rescue of wild-type Elovl4 expression (S ⁺ Elovl4 ^mut/mut mice) develop seizures by P19 and die by P21. Electrophysiological analyses of hippocampal slices showed aberrant epileptogenic activity in S ⁺ Elovl4 ^mut/mut mice. FM1-43 dye release studies showed that synapses made by cultured hippocampal neurons from S ⁺ Elovl4 ^mut/mut mice exhibited accelerated synaptic release kinetics. Supplementation of VLC-SFA to cultured hippocampal neurons from mutant mice rescued defective synaptic release to wild-type rates. Together, these studies establish a critical, novel role for ELOVL4 and its VLC-SFA products in regulating synaptic release kinetics and epileptogenesis. Future studies aimed at understanding the molecular mechanisms by which VLC-SFA regulate synaptic function may provide new targets for improved seizure therapies.

Sphingosine Kinase 1 Cooperates with Autophagy to Maintain Endocytic Membrane Trafficking

Sphingosine kinase 1 (Sphk1) associates with early endocytic membranes during endocytosis; however, the role of sphingosine or sphingosine-1-phosphate as the critical metabolite in endocytic trafficking has not been established. Here, we demonstrate that the recruitment of Sphk1 to sphingosine-enriched endocytic vesicles and the generation of sphingosine-1-phosphate facilitate membrane trafficking along the endosomal pathway. Exogenous sphingosine and sphingosine-based Sphk1 inhibitors induce the Sphk1-dependent fusion of endosomal membranes to accumulate enlarged late endosomes and amphisomes enriched in sphingolipids. Interestingly, Sphk1 also appears to facilitate endosomal fusion independent of its catalytic activity, given that catalytically inactive Sphk1^G82D is recruited to endocytic membranes by sphingosine or sphingosine-based Sphk1 inhibitor and promotes membrane fusion. Furthermore, we reveal that the clearance of enlarged endosomes is dependent on the activity of ceramide synthase, lysosomal biogenesis, and the restoration of autophagic flux. Collectively, these studies uncover intersecting roles for Sphk1, sphingosine, and autophagic machinery in endocytic membrane trafficking.

Clomipramine counteracts lipid raft disturbance due to short-term muscle disuse

Disuse-induced skeletal muscle dysfunction is a serious consequence of long-term spaceflight, numerous diseases and conditions for which treatment possibilities are still strictly limited. We have previously shown that acute hindlimb suspension (HS)-mediated disuse disrupts membrane lipid rafts in the unloaded muscle. Here, we investigated whether pretreatment of rats with the inhibitor of acid sphingomyelinase, clomipramine (1.25mg/g/day, intramuscularly, for 5days before HS), is able to hinder the loss in lipid raft integrity in response to 12h of HS. Clomipramine pretreatment significantly counteracted the decrease in labeling of the plasma membranes with lipid raft markers (fluorescent cholera toxin B subunit and bodipy-GM1-ganglioside) specifically in the junctional regions of the suspended soleus muscle. This was associated with: a) enhancing raft disrupting potential of exogenous sphingomyelinase in the junctional membranes; b) prevention of both ceramide accumulation and cholesterol loss; c) prevention of decline in nicotinic acetylcholine receptor labeling in the unloaded muscle. Our data suggest that sphingomyelinase-mediated raft disturbance serves as one of the earlier events in HS effects.

Lipid metabolic perturbation is an early-onset phenotype in adult spinster mutants: a Drosophila model for lysosomal storage disorders

Intracellular accumulation of lipids and swollen dysfunctional lysosomes are linked to several neurodegenerative diseases, including lysosomal storage disorders (LSD). Detailed characterization of lipid metabolic changes in relation to the onset and progression of neurodegeneration is currently missing. We systematically analyzed lipid perturbations in spinster (spin) mutants, a Drosophila model of LSD-like neurodegeneration. Our results highlight an imbalance in brain ceramide and sphingosine in the early stages of neurodegeneration, preceding the accumulation of endomembranous structures, manifestation of altered behavior, and buildup of lipofuscin. Manipulating levels of ceramidase and altering these lipids in spin mutants allowed us to conclude that ceramide homeostasis is the driving force in disease progression and is integral to spin function in the adult nervous system. We identified 29 novel physical interaction partners of Spin and focused on the lipid carrier protein, Lipophorin (Lpp). A subset of Lpp and Spin colocalize in the brain and within organs specialized for lipid metabolism (fat bodies and oenocytes). Reduced Lpp protein was observed in spin mutant tissues. Finally, increased levels of lipid metabolites produced by oenocytes in spin mutants allude to a functional interaction between Spin and Lpp, underscoring the systemic nature of lipid perturbation in LSD.

FRMD4A-cytohesin signaling modulates the cellular release of tau

One of the defining pathological features of Alzheimer's disease is the intraneuronal accumulation of tau (also known as MAPT) protein. Tau is also secreted from neurons in response to various stimuli and accumulates in the cerebrospinal fluid of Alzheimer's disease patients. Tau pathology might spread from cell to cell through a mechanism involving secretion and uptake. Here, we developed an assay to follow cellular release and uptake of tau dimers. Individual silencing of ten common late-onset Alzheimer's disease risk genes in HEK293T cells expressing the tau reporters suggested that FRMD4A is functionally linked to tau secretion. FRMD4A depletion by using RNA interference (RNAi) reduced and overexpression increased tau secretion. The activity of cytohesins, interactors of FRMD4A and guanine-nucleotide-exchange factors of Arf6, was necessary for FRMD4A-induced tau secretion. Increased Arf6 and cell polarity signaling through Par6 and atypical protein kinase Cζ (aPKCζ) stimulated tau secretion. In mature cortical neurons, FRMD4A RNAi or inhibition of cytohesins strongly upregulated secretion of endogenous tau. These results suggest that FRMD4A, a genetic risk factor for late-onset Alzheimer's disease, regulates tau secretion by activating cytohesin-Arf6 signaling. We conclude that genetic risk factors of Alzheimer's disease might modulate disease progression by altering tau secretion.

A New Fiji-Based Algorithm That Systematically Quantifies Nine Synaptic Parameters Provides Insights into Drosophila NMJ Morphometry

The morphology of synapses is of central interest in neuroscience because of the intimate relation with synaptic efficacy. Two decades of gene manipulation studies in different animal models have revealed a repertoire of molecules that contribute to synapse development. However, since such studies often assessed only one, or at best a few, morphological features at a given synapse, it remained unaddressed how different structural aspects relate to one another. Furthermore, such focused and sometimes only qualitative approaches likely left many of the more subtle players unnoticed. Here, we present the image analysis algorithm 'Drosophila_NMJ_Morphometrics', available as a Fiji-compatible macro, for quantitative, accurate and objective synapse morphometry of the Drosophila larval neuromuscular junction (NMJ), a well-established glutamatergic model synapse. We developed this methodology for semi-automated multiparametric analyses of NMJ terminals immunolabeled for the commonly used markers Dlg1 and Brp and showed that it also works for Hrp, Csp and Syt. We demonstrate that gender, genetic background and identity of abdominal body segment consistently and significantly contribute to variability in our data, suggesting that controlling for these parameters is important to minimize variability in quantitative analyses. Correlation and principal component analyses (PCA) were performed to investigate which morphometric parameters are inter-dependent and which ones are regulated rather independently. Based on nine acquired parameters, we identified five morphometric groups: NMJ size, geometry, muscle size, number of NMJ islands and number of active zones. Based on our finding that the parameters of the first two principal components hardly correlated with each other, we suggest that different molecular processes underlie these two morphometric groups. Our study sets the stage for systems morphometry approaches at the well-studied Drosophila NMJ.

Ceramides And Stress Signalling Intersect With Autophagic Defects In Neurodegenerative Drosophila blue cheese (bchs) Mutants

Sphingolipid metabolites are involved in the regulation of autophagy, a degradative recycling process that is required to prevent neuronal degeneration. Drosophila blue cheese mutants neurodegenerate due to perturbations in autophagic flux, and consequent accumulation of ubiquitinated aggregates. Here, we demonstrate that blue cheese mutant brains exhibit an elevation in total ceramide levels; surprisingly, however, degeneration is ameliorated when the pool of available ceramides is further increased, and exacerbated when ceramide levels are decreased by altering sphingolipid catabolism or blocking de novo synthesis. Exogenous ceramide is seen to accumulate in autophagosomes, which are fewer in number and show less efficient clearance in blue cheese mutant neurons. Sphingolipid metabolism is also shifted away from salvage toward de novo pathways, while pro-growth Akt and MAP pathways are down-regulated, and ER stress is increased. All these defects are reversed under genetic rescue conditions that increase ceramide generation from salvage pathways. This constellation of effects suggests a possible mechanism whereby the observed deficit in a potentially ceramide-releasing autophagic pathway impedes survival signaling and exacerbates neuronal death.

Ceramides in Alzheimer's Disease: Key Mediators of Neuronal Apoptosis Induced by Oxidative Stress and Aβ Accumulation

Alzheimer's disease (AD), the most common chronic and progressive neurodegenerative disorder, is characterized by extracellular deposits of amyloid β-peptides (Aβ) and intracellular deposits of hyperphosphorylated tau (phospho-tau) protein. Ceramides, the major molecules of sphingolipid metabolism and lipid second messengers, have been associated with AD progression and pathology via Aβ generation. Enhanced levels of ceramides directly increase Aβ through stabilization of β-secretase, the key enzyme in the amyloidogenic processing of Aβ precursor protein (APP). As a positive feedback loop, the generated oligomeric and fibrillar Aβ induces a further increase in ceramide levels by activating sphingomyelinases that catalyze the catabolic breakdown of sphingomyelin to ceramide. Evidence also supports important role of ceramides in neuronal apoptosis. Ceramides may initiate a cascade of biochemical alterations, which ultimately leads to neuronal death by diverse mechanisms, including depolarization and permeabilization of mitochondria, increased production of reactive oxygen species (ROS), cytochrome c release, Bcl-2 depletion, and caspase-3 activation, mainly by modulating intracellular signalling, particularly along the pathways related to Akt/PKB kinase and mitogen-activated protein kinases (MAPKs). This review summarizes recent findings related to the role of ceramides in oxidative stress-driven neuronal apoptosis and interplay with Aβ in the cascade of events ending in neuronal degeneration.

Sphingolipid metabolism correlates with cerebrospinal fluid Beta amyloid levels in Alzheimer's disease

Sphingolipids are important in many brain functions but their role in Alzheimer’s disease (AD) is not completely defined. A major limit is availability of fresh brain tissue with defined AD pathology. The discovery that cerebrospinal fluid (CSF) contains abundant nanoparticles that include synaptic vesicles and large dense core vesicles offer an accessible sample to study these organelles, while the supernatant fluid allows study of brain interstitial metabolism. Our objective was to characterize sphingolipids in nanoparticles representative of membrane vesicle metabolism, and in supernatant fluid representative of interstitial metabolism from study participants with varying levels of cognitive dysfunction. We recently described the recruitment, diagnosis, and CSF collection from cognitively normal or impaired study participants. Using liquid chromatography tandem mass spectrometry, we report that cognitively normal participants had measureable levels of sphingomyelin, ceramide, and dihydroceramide species, but that their distribution differed between nanoparticles and supernatant fluid, and further differed in those with cognitive impairment. In CSF from AD compared with cognitively normal participants: a) total sphingomyelin levels were lower in nanoparticles and supernatant fluid; b) levels of ceramide species were lower in nanoparticles and higher in supernatant fluid; c) three sphingomyelin species were reduced in the nanoparticle fraction. Moreover, three sphingomyelin species in the nanoparticle fraction were lower in mild cognitive impairment compared with cognitively normal participants. The activity of acid, but not neutral sphingomyelinase was significantly reduced in the CSF from AD participants. The reduction in acid sphingomylinase in CSF from AD participants was independent of depression and psychotropic medications. Acid sphingomyelinase activity positively correlated with amyloid β₄₂ concentration in CSF from cognitively normal but not impaired participants. In dementia, altered sphingolipid metabolism, decreased acid sphingomyelinase activity and its lost association with CSF amyloid β₄₂ concentration, underscores the potential of sphingolipids as disease biomarkers, and acid sphingomyelinase as a target for AD diagnosis and/or treatment.

Genetics of gene expression characterizes response to selective breeding for alcohol preference

Numerous selective breeding experiments have been performed with rodents, in an attempt to understand the genetic basis for innate differences in preference for alcohol consumption. Quantitative trait locus (QTL) analysis has been used to determine regions of the genome that are associated with the behavioral difference in alcohol preference/consumption. Recent work suggests that differences in gene expression represent a major genetic basis for complex traits. Therefore, the QTLs are likely to harbor regulatory regions (eQTLs) for the differentially expressed genes that are associated with the trait. In this study, we examined brain gene expression differences over generations of selection of the third replicate lines of high and low alcohol-preferring (HAP3 and LAP3) mice, and determined regions of the genome that control the expression of these differentially expressed genes (de eQTLs). We also determined eQTL regions (rv eQTLs) for genes that showed a decrease in variance of expression levels over the course of selection. We postulated that de eQTLs that overlap with rv eQTLs, and also with phenotypic QTLs, represent genomic regions that are affected by the process of selection. These overlapping regions controlled the expression of candidate genes (that displayed differential expression and reduced variance of expression) for the predisposition to differences in alcohol consumption by the HAP3/LAP3 mice.

Diacylglycerol, phosphatidic acid, and their metabolic enzymes in synaptic vesicle recycling

The synaptic vesicle (SV) cycle includes exocytosis of vesicles loaded with a neurotransmitter such as glutamate, coordinated recovery of SVs by endocytosis, refilling of vesicles, and subsequent release of the refilled vesicles from the presynaptic bouton. SV exocytosis is tightly linked with endocytosis, and variations in the number of vesicles, and/or defects in the refilling of SVs, will affect the amount of neurotransmitter available for release (Sudhof, 2004). There is increasing interest in the roles synaptic vesicle lipids and lipid metabolizing enzymes play in this recycling. Initial emphasis was placed on the role of polyphosphoinositides in SV cycling as outlined in a number of reviews (Lim and Wenk, 2009; Martin, 2012; Puchkov and Haucke, 2013; Rohrbough and Broadie, 2005). Other lipids are now recognized to also play critical roles. For example, PLD1 (Humeau et al., 2001; Rohrbough and Broadie, 2005) and some DGKs (Miller et al., 1999; Nurrish et al., 1999) play roles in neurotransmission which is consistent with the critical roles for phosphatidic acid (PtdOH) and diacylglycerol (DAG) in the regulation of SV exo/endocytosis (Cremona et al., 1999; Exton, 1994; Huttner and Schmidt, 2000; Lim and Wenk, 2009; Puchkov and Haucke, 2013; Rohrbough and Broadie, 2005). PLD generates phosphatidic acid by catalyzing the hydrolysis of phosphatidylcholine (PtdCho) and in some systems this PtdOH is de-phosphorylated to generate DAG. In contrast, DGK catalyzes the phosphorylation of DAG thereby converting it into PtdOH. While both enzymes are poised to regulate the levels of DAG and PtdOH, therefore, they both lead to the generation of PtdOH and could have opposite effects on DAG levels. This is particularly important for SV cycling as PtdOH and DAG are both needed for evoked exocytosis (Lim and Wenk, 2009; Puchkov and Haucke, 2013; Rohrbough and Broadie, 2005). Two lipids and their involved metabolic enzymes, two sphingolipids have also been implicated in exocytosis: sphingosine (Camoletto et al., 2009; Chan et al., 2012; Chan and Sieburth, 2012; Darios et al., 2009; Kanno et al., 2010; Rohrbough et al., 2004) and sphingosine-1-phosphate (Chan, Hu, 2012; Chan and Sieburth, 2012; Kanno et al., 2010). Finally a number of reports have focused on the somewhat less well studies roles of sphingolipids and cholesterol in SV cycling. In this report, we review the recent understanding of the roles PLDs, DGKs, and DAG lipases, as well as sphingolipids and cholesterol play in synaptic vesicle cycling.

Coupling between endocytosis and sphingosine kinase 1 recruitment

Genetic studies have suggested a functional link between cholesterol/sphingolipid metabolism and endocytic membrane traffic. Here we show that perturbing the cholesterol/sphingomyelin balance in the plasma membrane results in the massive formation of clusters of narrow endocytic tubular invaginations positive for N-BAR proteins. These tubules are intensely positive for sphingosine kinase 1 (SPHK1). SPHK1 is also targeted to physiologically occurring early endocytic intermediates, and is highly enriched in nerve terminals, which are cellular compartments specialized for exo/endocytosis. Membrane recruitment of SPHK1 involves a direct, curvature-sensitive interaction with the lipid bilayer mediated by a hydrophobic patch on the enzyme's surface. The knockdown of SPHKs results in endocytic recycling defects, and a mutation that disrupts the hydrophobic patch of Caenorhabditis elegans SPHK fails to rescue the neurotransmission defects in loss-of-function mutants of this enzyme. Our studies support a role for sphingosine phosphorylation in endocytic membrane trafficking beyond the established function of sphingosine-1-phosphate in intercellular signalling.

Endocytosis and intracellular processing of BODIPY-sphingomyelin by murine CATH.a neurons

Neuronal sphingolipids (SL) play important roles during axonal extension, neurotrophic receptor signaling and neurotransmitter release. Many of these signaling pathways depend on the presence of specialized membrane microdomains termed lipid rafts. Sphingomyelin (SM), one of the main raft constituents, can be formed de novo or supplied from exogenous sources. The present study aimed to characterize fluorescently-labeled SL turnover in a murine neuronal cell line (CATH.a). Our results demonstrate that at 4°C exogenously added BODIPY-SM accumulates exclusively at the plasma membrane. Treatment of cells with bacterial sphingomyelinase (SMase) and back-exchange experiments revealed that 55-67% of BODIPY-SM resides in the outer leaflet of the plasma membrane. Endocytosis of BODIPY-SM occurs via caveolae with part of internalized BODIPY-fluorescence ending up in the Golgi and the ER. Following endocytosis BODIPY-SM undergoes hydrolysis, a reaction substantially faster than BODIPY-SM synthesis from BODIPY-ceramide. RNAi demonstrated that both, acid (a)SMase and neutral (n)SMases contribute to BODIPY-SM hydrolysis. Finally, high-density lipoprotein (HDL)-associated BODIPY-SM was efficiently taken up by CATH.a cells. Our findings indicate that endocytosis of exogenous SM occurs almost exclusively via caveolin-dependent pathways, that both, a- and nSMases equally contribute to neuronal SM turnover and that HDL-like particles might represent physiological SM carriers/donors in the brain.

Sphingolipid regulation of ezrin, radixin, and moesin proteins family: implications for cell dynamics

A key but poorly studied domain of sphingolipid functions encompasses endocytosis, exocytosis, cellular trafficking, and cell movement. Recently, the ezrin, radixin and moesin (ERM) family of proteins emerged as novel potent targets regulated by sphingolipids. ERMs are structural proteins linking the actin cytoskeleton to the plasma membrane, also forming a scaffold for signaling pathways that are used for cell proliferation, migration and invasion, and cell division. Opposing functions of the bioactive sphingolipid ceramide and sphingosine-1-phosphate (S1P), contribute to ERM regulation. S1P robustly activates whereas ceramide potently deactivates ERM via phosphorylation/dephosphorylation, respectively. This recent dimension of cytoskeletal regulation by sphingolipids opens up new avenues to target cell dynamics, and provides further understanding of some of the unexplained biological effects mediated by sphingolipids. In addition, these studies are providing novel inroads into defining basic mechanisms of regulation and action of bioactive sphingolipids. This review describes the current understanding of sphingolipid regulation of the cytoskeleton, it also describes the biologies in which ERM proteins have been involved, and finally how these two large fields have started to converge. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Survival response to increased ceramide involves metabolic adaptation through novel regulators of glycolysis and lipolysis

The sphingolipid ceramide elicits several stress responses, however, organisms survive despite increased ceramide but how they do so is poorly understood. We demonstrate here that the AKT/FOXO pathway regulates survival in increased ceramide environment by metabolic adaptation involving changes in glycolysis and lipolysis through novel downstream targets. We show that ceramide kinase mutants accumulate ceramide and this leads to reduction in energy levels due to compromised oxidative phosphorylation. Mutants show increased activation of Akt and a consequent decrease in FOXO levels. These changes lead to enhanced glycolysis by upregulating the activity of phosphoglyceromutase, enolase, pyruvate kinase, and lactate dehydrogenase to provide energy. A second major consequence of AKT/FOXO reprogramming in the mutants is the increased mobilization of lipid from the gut through novel lipase targets, CG8093 and CG6277 for energy contribution. Ubiquitous reduction of these targets by knockdown experiments results in semi or total lethality of the mutants, demonstrating the importance of activating them. The efficiency of these adaptive mechanisms decreases with age and leads to reduction in adult life span of the mutants. In particular, mutants develop cardiac dysfunction with age, likely reflecting the high energy requirement of a well-functioning heart. The lipases also regulate physiological triacylglycerol homeostasis and are important for energy metabolism since midgut specific reduction of them in wild type flies results in increased sensitivity to starvation and accumulation of triglycerides leading to cardiac defects. The central findings of increased AKT activation, decreased FOXO level and activation of phosphoglyceromutase and pyruvate kinase are also observed in mice heterozygous for ceramide transfer protein suggesting a conserved role of this pathway in mammals. These data reveal novel glycolytic and non-autonomous lipolytic pathways in response to increased ceramide for sustenance of high energy demanding organ functions like the heart.

Sphingolipids in psychiatric disorders and pain syndromes

Despite the high prevalence and devastating impact of psychiatric disorders, little is known about their etiopathology. In this review, we provide an overview on the participation of sphingolipids and enzymes responsible for their metabolism in mechanisms underlying psychiatric disorders. We focus on the pathway from sphingomyelin to proapoptotic ceramide and the subsequent metabolism of ceramide to sphingosine, which is in turn phosphorylated to yield anti-apoptotic sphingosine-1-phosphate (S1P).The sphingomyelinase/ceramide system has been linked to effects of reactive oxygen species and proinflammatory cytokines in the central nervous system as well as to synaptic transmission. Compared to ubiquitously expressed acid sphingomyelinase, acid and neutral ceramidase and neutral sphingomyelinase are highly active in brain regions. Depressed patients show elevated plasma ceramide levels and increased activities of acid sphingomyelinase which is functionally inhibited by many anti-depressive drugs. Exposure to alcohol is associated with an activation of acid and neutral sphingomyelinase observed in cell culture, mouse models and in alcohol-dependent patients and with increased concentrations of ceramide in various organs.Levels of sphingomyelin and ceramide are altered in erythrocytes and post-mortem brain tissues of schizophrenic patients in addition to changes in expression patterns for serine palmitoyltransferase and acid ceramidase leading to impaired myelination. After induction of anxiety-like behavior in animal models, higher serum levels of S1P were reported to lead to neurodegeneration. Correspondingly, S1P infusion appeared to increase anxiety-like behavior. Significantly upregulated levels of the endogenous ceramide catabolite N,N-dimethylsphingosine were observed in rat models of allodynia. Conversely, rats injected intrathecally with N,N-dimethylsphingosine developed mechanical allodynia. Moreover, S1P has been implicated in spinal nociceptive processing.The increasing interest in lipidomics and improved analytical methods led to growing insight into the connection between psychiatric and neurological disorders and sphingolipid metabolism and may once provide new targets and strategies for therapeutic intervention.

Recruitment of sphingosine kinase to presynaptic terminals by a conserved muscarinic signaling pathway promotes neurotransmitter release

Sphingolipids are potent lipid second messengers that regulate cell differentiation, migration, survival, and secretion, and alterations in sphingolipid signaling have been implicated in a variety of diseases. However, how sphingolipid levels are regulated, particularly in the nervous system, remains poorly understood. Here, we show that the generation of sphingosine-1-phosphate by sphingosine kinase (SphK) promotes neurotransmitter release. Electrophysiological, imaging, and behavioral analyses of Caenorhabditis elegans mutants lacking sphingosine kinase sphk-1 indicate that neuronal development is normal, but there is a significant defect in neurotransmitter release from neuromuscular junctions. SPHK-1 localizes to discrete, nonvesicular regions within presynaptic terminals, and this localization is critical for synaptic function. Muscarinic agonists cause a rapid increase in presynaptic SPHK-1 abundance, whereas reduction of endogenous acetylcholine production results in a rapid decrease in presynaptic SPHK-1 abundance. Muscarinic regulation of presynaptic SPHK-1 abundance is mediated by a conserved presynaptic signaling pathway composed of the muscarinic acetylcholine receptor GAR-3, the heterotrimeric G protein Gαq, and its effector, Trio RhoGEF. SPHK-1 activity is required for the effects of muscarinic signaling on synaptic transmission. This study shows that SPHK-1 promotes neurotransmitter release in vivo and identifies a novel muscarinic pathway that regulates SphK abundance at presynaptic terminals.

Structure-function analysis of endogenous lectin mind-the-gap in synaptogenesis

Mind-the-Gap (MTG) is required for neuronal induction of Drosophila neuromuscular junction (NMJ) postsynaptic domains, including glutamate receptor (GluR) localization. We have previously hypothesized that MTG is secreted from the presynaptic terminal to reside in the synaptic cleft, where it binds glycans to organize the heavily glycosylated, extracellular synaptomatrix required for transsynaptic signaling between neuron and muscle. In this study, we test this hypothesis with MTG structure-function analyses of predicted signal peptide (SP) and carbohydrate-binding domain (CBD), by introducing deletion and point-mutant transgenic constructs into mtg null mutants. We show that the SP is required for MTG secretion and localization to synapses in vivo. We further show that the CBD is required to restrict MTG diffusion in the extracellular synaptomatrix and for postembryonic viability. However, CBD mutation results in elevation of postsynaptic GluR localization during synaptogenesis, not the mtg null mutant phenotype of reduced GluRs as predicted by our hypothesis, suggesting that proper synaptic localization of MTG limits GluR recruitment. In further testing CBD requirements, we show that MTG binds N-acetylglucosamine (GlcNAc) in a Ca(2+)-dependent manner, and thereby binds HRP-epitope glycans, but that these carbohydrate interactions do not require the CBD. We conclude that the MTG lectin has both positive and negative binding interactions with glycans in the extracellular synaptic domain, which both facilitate and limit GluR localization during NMJ embryonic synaptogenesis.

The Role of ATP-Binding Cassette Transporters in Neuro-Inflammation: Relevance for Bioactive Lipids

ATP-binding cassette (ABC) transporters are highly expressed by brain endothelial cells that form the blood–brain barrier (BBB). These efflux pumps play an important role in maintaining brain homeostasis as they actively hinder the entry of unwanted blood-derived compounds into the central nervous system (CNS). Consequently, their high activity at the BBB has been a major hurdle for the treatment of several brain diseases, as they prevent numerous drugs to reach their site of action within the brain. Importantly, recent data indicate that endogenous substrates for ABC transporters may include inflammatory mediators, such as prostaglandins, leukotrienes, cytokines, chemokines, and bioactive lipids, suggesting a potential role for ABC transporters in immunological responses, and more specifically in inflammatory brain disorders, such as multiple sclerosis (MS). In this review, we will give a comprehensive overview of recent findings that illustrate this novel role for ABC transporters in neuro-inflammatory processes. Moreover, we will provide first insights into underlying mechanisms and focus on the importance for bioactive lipids, in particular platelet-activating factor, herein. A thorough understanding of these events may form the basis for the development for selective treatment modalities to dampen the neuro-inflammatory attack in MS and thereby reducing tissue damage.

A genetic analysis of the stinging and guarding behaviors of the honey bee

In order to identify genes that are influencing defensive behaviors, we have taken a new approach by dissecting colony-level defensive behavior into individual behavioral measurements using two families containing backcross workers from matings involving European and Africanized bees. We removed the social context from stinging behavior by using a laboratory assay to measure the stinging response of individual bees. A mild shock was given to bees using a constant-current stimulator. The time it took bees to sting in response to this stimulus was recorded. In addition, bees that were seen performing guard behaviors at the hive entrance were collected. We performed QTL mapping in two backcross families with SNP probes within genes and identified two new QTL regions for stinging behavior and another QTL region for guarding behavior. We also identified several candidate genes involved in neural signaling, neural development and muscle development that may be influencing stinging and guarding behaviors. The lack of overlap between these regions and previous defensive behavior QTL underscores the complexity of this behavior and increases our understanding of its genetic architecture.

C(6)-ceramide enhances phagocytic activity of Kupffer cells through the production of endogenous ceramides

Ceramide has been suggested to be not only a tumor-suppressive lipid but also a regulator of phagocytosis. We examined whether exogenous cell-permeable C(6)-ceramide enhances the phagocytic activity of Kupffer cells (KCs) and affects the level of cellular ceramides. Rat KCs were isolated by collagenase digestion and differential centrifugation, using Percoll system. Phagocytic activity was measured by FACS analysis after incubating KCs with fluorescence-conjugated latex beads, and the level of cellular ceramide was analyzed by liquid chromatography tandem-mass spectrometry (LC-MS/MS). In this study we found that permeable C(6)-ceramide increases the cellular levels of endogenous ceramides via a sphingosine-recycling pathway leading to enhanced phagocytosis by KCs.

Roles of sphingolipids in Drosophila development and disease

The last 10 years have seen a rebirth of interest in lipid biology in the fields of Drosophila development and neurobiology, and sphingolipids have emerged as controlling many processes that have not previously been studied from the viewpoint of lipid biochemistry. Mutations in sphingolipid regulatory enzymes have been pinpointed as affecting cell survival and growth in tissues ranging from muscle to retina. Specification of cell types are also influenced by sphingolipid regulatory pathways, as genetic interactions of glycosphingolipid biosynthetic enzymes with many well-known signaling receptors such as Notch and epidermal growth factor receptor reveal. Furthermore, studies in flies are now uncovering unexpected roles of sphingolipids in controlling lipid storage and response to nutrient availability. The sophisticated genetics of Drosophila is particularly well suited to uncover the roles of sphingolipid regulatory enzymes in development and metabolism, especially in light of conserved pathways that are present in both flies and mammals. The challenges that remain in the field of sphingolipid biology in Drosophila are to combine traditional developmental genetics with more analytical biochemical and biophysical methods, to quantify and localize the responses of these lipids to genetic and metabolic perturbations.

CDase is a pan-ceramidase in Drosophila

It is demonstrated that the Cdase gene encodes for all measurable ceramidase function in Drosophila. BWA, an alkaline ceramidase homologue, does not exhibit ceramidase activity. Bwa genetically interacts with other ceramide-metabolizing enzymes by influencing the flux through the sphingolipid pathway.

 Ceramidases catalyze the conversion of ceramide to sphingosine. They are acylaminohydrolases that catalyze the deacylation of the amide-linked saturated fatty acid from ceramide to generate sphingosine. They also catalyze the reverse reaction of ceramide biosynthesis using sphingosine and fatty acid. In mammals, different proteins catalyze these reactions while individually exhibiting optimal activity over a narrow pH range and have been accordingly called acid, neutral, and alkaline ceramidases. Several genes encode for variants of alkaline ceramidase in mammals. Brainwashing (Bwa) is the only putative alkaline ceramidase homologue present in Drosophila. In this study we have demonstrated that BWA does not exhibit ceramidase activity and that bwa null mutants display no loss of ceramidase activity. Instead, the neutral ceramidase gene CDase encodes the protein that is responsible for all measurable ceramidase activity in Drosophila. Our studies show strong genetic interaction of Bwa with CDase and the Drosophila ceramide kinase gene (DCERK). We show that, although BWA is unlikely to be a ceramidase, it is a regulator of sphingolipid flux in Drosophila. Bwa exhibits strong genetic interaction with other genes coding for ceramide-metabolizing enzymes. This interaction might partly explain its original identification as a ceramidase.

Inhibition of neutral sphingomyelinase-2 perturbs brain sphingolipid balance and spatial memory in mice

The sphingolipid ceramide is a bioactive signaling lipid that is thought to play important roles in modulating synaptic activity, in part by regulating the function of excitatory postsynaptic receptors. However, the molecular mechanisms by which ceramide exerts its effects on synaptic activity remain largely unknown. We recently demonstrated that a rapid generation of ceramide by neutral sphingomyelinase-2 (nSMase2; also known as "sphingomyelin phosphodiesterase-3") played a key role in modulating excitatory postsynaptic currents by controlling the insertion and clustering of NMDA receptors (Wheeler et al. [2009] J. Neurochem. 109:1237-1249). We now demonstrate that nSMase2 plays a role in memory. Inhibition of nSMase2 impaired spatial and episodic-like memory in mice. At the molecular level, inhibition of nSMase2 decreased ceramide, increased PSD-95, increased the number of AMPA receptors, and altered the subunit composition of NMDA receptors. Our study identifies nSMase2 as an important component for efficient memory formation and underscores the importance of ceramide in regulating synaptic events related to learning and memory.