Ceramides modulate cell-surface acetylcholine receptor levels

Diet-Induced Obesity in the Rat Impairs Sphingolipid Metabolism in the Brain and This Metabolic Dysfunction Is Transmitted to the Offspring via Both the Maternal and the Paternal Lineage

Obesity leads to a number of health problems, including learning and memory deficits that can be passed on to the offspring via a developmental programming process. However, the mechanisms involved in the deleterious effects of obesity on cognition remain largely unknown. This study aimed to assess the impact of obesity on the production of sphingolipids (ceramides and sphingomyelins) in the brain and its relationship with the learning deficits displayed by obese individuals. We also sought to determine whether the effects of obesity on brain sphingolipid synthesis could be passed on to the offspring. Learning abilities and brain concentration of sphingolipids in male and female control and obese founder rats (F0) and their offspring (F1) were evaluated, respectively, by the novel object recognition test and by ultra-performance liquid chromatography tandem mass spectrometry. In addition, a global lipidome profiling of the cerebral cortex and hippocampus was performed. Both male and female F0 rats showed impaired learning and increased concentrations of ceramides and sphingomyelins in the hippocampus and frontal cortex compared to their control counterparts. However, the overall lipidome profile of these brain regions did not change with obesity. Remarkably, the alterations in brain sphingolipid synthesis, as well as the cognitive impairment induced by obesity, were also present in adult F1 male rats born to obese mothers or sired by obese fathers and were associated with enhanced expression of mRNAs coding for enzymes involved in the de novo synthesis of ceramides. These results show that the cognitive deficits and impaired sphingolipid metabolism induced by obesity can be transmitted to the offspring through both the maternal and paternal lineages and suggest that an increase in the brain concentration of sphingolipids could play a causal role in the cognitive deficits associated with obesity.

Bioactive sphingolipids as emerging targets for signal transduction in cancer development

Sphingolipids, crucial components of cellular membranes, play a vital role in maintaining cellular structure and signaling integrity. Disruptions in sphingolipid metabolism are increasingly implicated in cancer development. Key bioactive sphingolipids, such as ceramides, sphingosine-1-phosphate (S1P), ceramide-1-phosphate (C1P), and glycosphingolipids, profoundly impact tumor biology. They influence the behavior of tumor cells, stromal cells, and immune cells, affecting tumor aggressiveness, angiogenesis, immune modulation, and extracellular matrix remodeling. Furthermore, abnormal expression of sphingolipids and their metabolizing enzymes modulates the secretion of tumor-derived extracellular vesicles (TDEs), which are key players in creating an immunosuppressive tumor microenvironment, remodeling the extracellular matrix, and facilitating oncogenic signaling within in situ tumors and distant pre-metastatic niches (PMNs). Understanding the role of sphingolipids in the biogenesis of tumor-derived extracellular vesicles (TDEs) and their bioactive contents can pave the way for new biomarkers in cancer diagnosis and prognosis, ultimately enhancing comprehensive tumor treatment strategies.

Identifying Membrane Protein-Lipid Interactions with Lipidomic Lipid Exchange-Mass Spectrometry

Lipids can play important roles in modulating membrane protein structure and function. However, it is challenging to identify natural lipids bound to membrane proteins in complex bilayers. Here, we developed lipidomic lipid exchange-mass spectrometry (LX-MS) to study the lipid affinity for membrane proteins on a lipidomic scale. We first mix membrane protein nanodiscs with empty nanodiscs that have no embedded membrane proteins. After allowing lipids to passively exchange between the two populations, we separate the two types of nanodiscs and perform lipidomic analysis on each with liquid chromatography and MS. Enrichment of lipids in the membrane protein nanodiscs reveals the affinity of individual lipids for binding the target membrane protein. We apply this approach to study three membrane proteins. With the Escherichia coli ammonium transporter AmtB and aquaporin AqpZ in nanodiscs with E. coli polar lipid extracts, we detected binding of cardiolipin and phosphatidyl-glycerol lipids to the proteins. With the acetylcholine receptor in nanodiscs with brain polar lipid extracts, we discovered a complex set of lipid interactions that depended on the head group and tail composition. Overall, lipidomic LX-MS provides a detailed understanding of the lipid-binding affinity and thermodynamics for membrane proteins in complex bilayers and provides a unique perspective on the chemical environment surrounding membrane proteins.

Interactions between the Nicotinic and Endocannabinoid Receptors at the Plasma Membrane

Compartmentalization, together with transbilayer and lateral asymmetries, provide the structural foundation for functional specializations at the cell surface, including the active role of the lipid microenvironment in the modulation of membrane-bound proteins. The chemical synapse, the site where neurotransmitter-coded signals are decoded by neurotransmitter receptors, adds another layer of complexity to the plasma membrane architectural intricacy, mainly due to the need to accommodate a sizeable number of molecules in a minute subcellular compartment with dimensions barely reaching the micrometer. In this review, we discuss how nature has developed suitable adjustments to accommodate different types of membrane-bound receptors and scaffolding proteins via membrane microdomains, and how this "effort-sharing" mechanism has evolved to optimize crosstalk, separation, or coupling, where/when appropriate. We focus on a fast ligand-gated neurotransmitter receptor, the nicotinic acetylcholine receptor, and a second-messenger G-protein coupled receptor, the cannabinoid receptor, as a paradigmatic example.

Effects of a moisturizer containing pseudo-ceramide and a eucalyptus extract on sweating function in adult atopic dermatitis: a double-blind, randomized, controlled left-right comparison clinical trial

BACKGROUND

Many patients with atopic dermatitis (AD) have a decreased ability to sweat. Several factors can cause decreased perspiration, such as weak tight junctions of sweat ducts, reduced acetylcholine receptor function and inhibition of perspiration by histamines. Parakeratosis of AD skin also decreases sweating by occluding sweat pores. Increased ceramide levels in the stratum corneum reduce parakeratosis by improving stratum corneum functions. Furthermore, ceramides and/or ceramide derivatives may affect claudin-3 and acetylcholine receptors.

OBJECTIVE

In this study, we investigated the efficacy of a moisturizer containing pseudo-ceramide and a eucalyptus extract to increase ceramide levels in the epidermis to improve the sweating ability of patients with AD.

METHODS

Nineteen patients with AD applied moisturizers with or without pseudo-ceramide and a eucalyptus extract on the cubital fossa of either arm twice a day for 4 weeks. Skin conditions and sweating ability, measured as the response to acetylcholine stimulation, were evaluated prior to the start of the study (week 0) and at the end of weeks 2 and 4.

RESULTS

Both moisturizers improved the visually evaluated skin symptoms and skin hydration. However, only the moisturizer containing pseudo-ceramide and the eucalyptus extract significantly improved cutaneous barrier function and significantly increased the ceramide level in the stratum corneum. That moisturizer also increased the sweating volume and shortened the latency time for sweating, an indicator of sweating ability, but the other moisturizer did not.

CONCLUSION

Based on these results, the moisturizer containing pseudo-ceramide and a eucalyptus extract helps recover the sweat function of AD patients.

Dysregulation of Neuronal Nicotinic Acetylcholine Receptor-Cholesterol Crosstalk in Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a set of complex neurodevelopmental diseases that include impaired social interaction, delayed and disordered language, repetitive or stereotypic behavior, restricted range of interests, and altered sensory processing. The underlying causes of the core symptoms remain unclear, as are the factors that trigger their onset. Given the complexity and heterogeneity of the clinical phenotypes, a constellation of genetic, epigenetic, environmental, and immunological factors may be involved. The lack of appropriate biomarkers for the evaluation of neurodevelopmental disorders makes it difficult to assess the contribution of early alterations in neurochemical processes and neuroanatomical and neurodevelopmental factors to ASD. Abnormalities in the cholinergic system in various regions of the brain and cerebellum are observed in ASD, and recently altered cholesterol metabolism has been implicated at the initial stages of the disease. Given the multiple effects of the neutral lipid cholesterol on the paradigm rapid ligand-gated ion channel, the nicotinic acetylcholine receptor, we explore in this review the possibility that the dysregulation of nicotinic receptor-cholesterol crosstalk plays a role in some of the neurological alterations observed in ASD.

Lithium causes differential effects on postsynaptic stability in normal and denervated neuromuscular synapses

Lithium chloride has been widely used as a therapeutic mood stabilizer. Although cumulative evidence suggests that lithium plays modulatory effects on postsynaptic receptors, the underlying mechanism by which lithium regulates synaptic transmission has not been fully elucidated. In this work, by using the advantageous neuromuscular synapse, we evaluated the effect of lithium on the stability of postsynaptic nicotinic acetylcholine receptors (nAChRs) in vivo. We found that in normally innervated neuromuscular synapses, lithium chloride significantly decreased the turnover of nAChRs by reducing their internalization. A similar response was observed in CHO-K1/A5 cells expressing the adult muscle-type nAChRs. Strikingly, in denervated neuromuscular synapses, lithium led to enhanced nAChR turnover and density by increasing the incorporation of new nAChRs. Lithium also potentiated the formation of unstable nAChR clusters in non-synaptic regions of denervated muscle fibres. We found that denervation-dependent re-expression of the foetal nAChR γ-subunit was not altered by lithium. However, while denervation inhibits the distribution of β-catenin within endplates, lithium-treated fibres retain β-catenin staining in specific foci of the synaptic region. Collectively, our data reveal that lithium treatment differentially affects the stability of postsynaptic receptors in normal and denervated neuromuscular synapses in vivo, thus providing novel insights into the regulatory effects of lithium on synaptic organization and extending its potential therapeutic use in conditions affecting the peripheral nervous system.

Sequential purification and characterization of Torpedo californica nAChR-DC supplemented with CHS for high-resolution crystallization studies

Over the past 10 years we have been developing a multi-attribute analytical platform that allows for the preparation of milligram amounts of functional, high-pure, and stable Torpedo (muscle-type) nAChR detergent complexes for crystallization purpose. In the present work, we have been able to significantly improve and optimize the purity and yield of nicotinic acetylcholine receptors in detergent complexes (nAChR-DC) without compromising stability and functionality. We implemented new methods in the process, such as analysis and rapid production of samples for future crystallization preparations. Native nAChR was extracted from the electric organ of Torpedo californica using the lipid-like detergent LysoFos Choline 16 (LFC-16), followed by three consecutive steps of chromatography purification. We evaluated the effect of cholesteryl hemisuccinate (CHS) supplementation during the affinity purification steps of nAChR-LFC-16 in terms of receptor secondary structure, stability and functionality. CHS produced significant changes in the degree of β-secondary structure, these changes compromise the diffusion of the nAChR-LFC-16 in lipid cubic phase. The behavior was reversed by Methyl-β-Cyclodextrin treatment. Also, CHS decreased acetylcholine evoked currents of Xenopus leavis oocyte injected with nAChR-LFC-16 in a concentration-dependent manner. Methyl-β-Cyclodextrin treatment do not reverse functionality, however column delipidation produced a functional protein similar to nAChR-LFC-16 without CHS treatment.

Acid sphingomyelinase - a regulator of canonical transient receptor potential channel 6 (TRPC6) activity

Recent investigations propose the acid sphingomyelinase (ASM)/ceramide system as a novel target for antidepressant action. ASM catalyzes the breakdown of the abundant membrane lipid sphingomyelin to the lipid messenger ceramide. This ASM-induced lipid modification induces a local shift in membrane properties, which influences receptor clustering and downstream signaling. Canonical transient receptor potential channels 6 (TRPC6) are non-selective cation channels located in the cell membrane that play an important role in dendritic growth, synaptic plasticity and cognition in the brain. They can be activated by hyperforin, an ingredient of the herbal remedy St. John's wort for treatment of depression disorders. Because of their role in the context of major depression, we investigated the crosstalk between the ASM/ceramide system and TRPC6 ion channels in a pheochromocytoma cell line 12 neuronal cell model (PC12 rat pheochromocytoma cell line). Ca²⁺ imaging experiments indicated that hyperforin-induced Ca²⁺ influx through TRPC6 channels is modulated by ASM activity. While antidepressants, known as functional inhibitors of ASM activity, reduced TRPC6-mediated Ca²⁺ influx, extracellular application of bacterial sphingomyelinase rebalanced TRPC6 activity in a concentration-related way. This effect was confirmed in whole-cell patch clamp electrophysiology recordings. Lipidomic analyses revealed a decrease in very long chain ceramide/sphingomyelin molar ratio after ASM inhibition, which was connected with changes in the abundance of TRPC6 channels in flotillin-1-positive lipid rafts as visualized by western blotting. Our data provide evidence that the ASM/ceramide system regulates TRPC6 channels likely by controlling their recruitment to specific lipid subdomains and thereby fine-tuning their physical properties.

Profiling of patient-specific myocytes identifies altered gene expression in the ophthalmoplegic subphenotype of myasthenia gravis

Background

While extraocular muscles are affected early in myasthenia gravis (MG), but respond to treatment, we observe a high incidence of treatment-resistant ophthalmoplegia (OP-MG) among MG subjects with African genetic ancestry. Previously, using whole exome sequencing, we reported potentially functional variants which associated with OP-MG. The aim of this study was to profile the expression of genes harbouring the OP-MG associated variants using patient-derived subphenotype-specific ‘myocyte’ cultures.

Methods

From well-characterised MG patients we developed the ‘myocyte’ culture models by transdifferentiating dermal fibroblasts using an adenovirus expressing MyoD. These myocyte cultures were treated with homologous acetylcholine receptor antibody-positive myasthenic sera to induce muscle transcripts in response to an MG stimulus. Gene expression in myocytes derived from OP-MG (n = 10) and control MG subjects (MG without ophthalmoplegia; n = 6) was quantified using a custom qPCR array profiling 93 potentially relevant genes which included the putative OP-MG susceptibility genes and other previously reported genes of interest in MG and experimental autoimmune myasthenia gravis (EAMG).

Results

OP-MG myocytes compared to control MG myocytes showed altered expression of four OP-MG susceptibility genes (PPP6R2, CANX, FAM136A and FAM69A) as well as several MG and EAMG genes (p < 0.05). A correlation matrix of gene pair expression levels revealed that 15% of gene pairs were strongly correlated in OP-MG samples (r > 0.78, p < 0.01), but not in control MG samples. OP-MG susceptibility genes and MG-associated genes accounted for the top three significantly correlated gene pairs (r ≥ 0.98, p < 1 × 10^− 6) reflecting crosstalk between OP-MG and myasthenia pathways, which was not evident in control MG cells. The genes with altered expression dynamics between the two subphenotypes included those with a known role in gangliosphingolipid biosynthesis, mitochondrial metabolism and the IGF1-signalling pathway.

Conclusion

Using a surrogate cell culture model our findings suggest that muscle gene expression and co-expression differ between OP-MG and control MG individuals. These findings implicate pathways not previously considered in extraocular muscle involvement in myasthenia gravis and will inform future studies.

Electronic supplementary material

The online version of this article (10.1186/s13023-019-1003-y) contains supplementary material, which is available to authorized users.

Exome sequencing identifies targets in the treatment-resistant ophthalmoplegic subphenotype of myasthenia gravis

Treatment-resistant ophthalmoplegia (OP-MG) is not uncommon in individuals with African genetic ancestry and myasthenia gravis (MG). To identify OP-MG susceptibility genes, extended whole exome sequencing was performed using extreme phenotype sampling (11 OP-MG vs 4 control-MG) all with acetylcholine receptor-antibody positive MG. This approach identified 356 variants that were twice as frequent in OP-MG compared to control-MG individuals. After performing probability test estimates and filtering variants according to those 'suggestive' of association with OP-MG (p < 0.05), only three variants remained which were expressed in extraocular muscles. Validation in 25 OP-MG and 50 control-MG cases supported the association of DDX17_delG (p = 0.014) and SPTLC3_insACAC (p = 0.055) with OP-MG, but ST8SIA1_delCCC could not be verified by Sanger sequencing. A parallel approach, using a semantic model informed by current knowledge of MG-pathways, identified an African-specific interleukin-6 receptor (IL6R) variant, IL6R c.*3043 T>C, that was more frequent in OP-MG compared to control-MG cases (p = 0.069) and population controls (p = 0.043). A weighted genetic risk score, derived from the odds ratios of association of these variants with OP-MG, correlated with the OP-MG phenotype as opposed to control MG. This unbiased approach implicates several potentially functional gene variants in the gangliosphingolipid and myogenesis pathways in the development of the OP-MG subphenotype.

Diacylglycerol levels modulate the cellular distribution of the nicotinic acetylcholine receptor

Diacylglycerol (DAG), a second messenger involved in different cell signaling cascades, activates protein kinase C (PKC) and D (PKD), among other kinases. The present work analyzes the effects resulting from the alteration of DAG levels on neuronal and muscle nicotinic acetylcholine receptor (AChR) distribution. We employ CHO-K1/A5 cells, expressing adult muscle-type AChR in a stable manner, and hippocampal neurons, which endogenously express various subtypes of neuronal AChR. CHO-K1/A5 cells treated with dioctanoylglycerol (DOG) for different periods showed augmented AChR cell surface levels at short incubation times (30min-4h) whereas at longer times (18h) the AChR was shifted to intracellular compartments. Similarly, in cultured hippocampal neurons surface AChR levels increased as a result of DOG incubation for 4h. Inhibition of endogenous DAG catabolism produced changes in AChR distribution similar to those induced by DOG treatment. Specific enzyme inhibitors and Western blot assays revealed that DAGs exert their effect on AChR distribution through the modulation of the activity of classical PKC (cPKC), novel PKC (nPKC) and PKD activity.

A Conserved Circular Network of Coregulated Lipids Modulates Innate Immune Responses

Lipid composition affects the biophysical properties of membranes that provide a platform for receptor-mediated cellular signaling. To study the regulatory role of membrane lipid composition, we combined genetic perturbations of sphingolipid metabolism with the quantification of diverse steps in Toll-like receptor (TLR) signaling and mass spectrometry-based lipidomics. Membrane lipid composition was broadly affected by these perturbations, revealing a circular network of coregulated sphingolipids and glycerophospholipids. This evolutionarily conserved network architecture simultaneously reflected membrane lipid metabolism, subcellular localization, and adaptation mechanisms. Integration of the diverse TLR-induced inflammatory phenotypes with changes in lipid abundance assigned distinct functional roles to individual lipid species organized across the network. This functional annotation accurately predicted the inflammatory response of cells derived from patients suffering from lipid storage disorders, based solely on their altered membrane lipid composition. The analytical strategy described here empowers the understanding of higher-level organization of membrane lipid function in diverse biological systems.

Nicotinic acetylcholine receptor-lipid interactions: Mechanistic insight and biological function

Membrane lipids are potent modulators of the nicotinic acetylcholine receptor (nAChR) from Torpedo. Lipids influence nAChR function by both conformational selection and kinetic mechanisms, stabilizing varying proportions of activatable versus non-activatable conformations, as well as influencing the transitions between these conformational states. Of note, some membranes stabilize an electrically silent uncoupled conformation that binds agonist but does not undergo agonist-induced conformational transitions. The uncoupled nAChR, however, does transition to activatable conformations in relatively thick lipid bilayers, such as those found in lipid rafts. In this review, we discuss current understanding of lipid-nAChR interactions in the context of increasingly available high resolution structural and functional data. These data highlight different sites of lipid action, including the lipid-exposed M4 transmembrane α-helix. Current evidence suggests that lipids alter nAChR function by modulating interactions between M4 and the adjacent transmembrane α-helices, M1 and M3. These interactions have also been implicated in both the folding and trafficking of nAChRs to the cell surface. We review current mechanistic understanding of lipid-nAChR interactions, and highlight potential biological roles for lipid-nAChR interactions in modulating the synaptic response. This article is part of a Special Issue entitled: Lipid-protein interactions.

Blockade of lysosomal acid ceramidase induces GluN2B-dependent Tau phosphorylation in rat hippocampal slices

The lysosomal acid ceramidase, an enzyme known to limit intracellular ceramide accumulation, has been reported to be defective in neurodegenerative disorders. We show here that rat hippocampal slices, preincubated with the acid ceramidase inhibitor (ACI) d-NMAPPD, exhibit increased N-methyl-D-aspartate (NMDA) receptor-mediated field excitatory postsynaptic potentials (fEPSPs) in CA1 synapses. The ACI by itself did not interfere with either paired pulse facilitation or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor-mediated fEPSPs, indicating that its influence on synaptic transmission is postsynaptic in origin and specific to the NMDA subtype of glutamate receptors. From a biochemical perspective, we observed that Tau phosphorylation at the Ser262 epitope was highly increased in hippocampal slices preincubated with the ACI, an effect totally prevented by the global NMDA receptor antagonist D/L(−)-2-amino-5-phosphonovaleric acid (AP-5), the calcium chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA), and the GluN2B (but not the GluN2A) receptor antagonist RO25-6981. On the other hand, preincubation of hippocampal slices with the compound KN-62, an inhibitor known to interfere with calcium/calmodulin-dependent protein kinase II (CaMKII), totally abolished the effect of ACI on Tau phosphorylation at Ser262 epitopes. Collectively, these results provide experimental evidence that ceramides play an important role in regulating Tau phosphorylation in the hippocampus via a mechanism dependent on GluN2B receptor subunits and CaMKII activation.

Antibody-induced acetylcholine receptor clusters inhabit liquid-ordered and liquid-disordered domains

The distribution of nicotinic acetylcholine receptor (AChR) clusters at the cell membrane was studied in CHO-K1/A5 cells using fluorescence microscopy. Di-4-ANEPPDHQ, a fluorescent probe that differentiates between liquid-ordered (Lo) and liquid-disordered (Ld) phases in model membranes, was used in combination with monoclonal anti-AChR antibody labeling of live cells, which induces AChR clustering. The so-called generalized polarization (GP) of di-4-ANEPPDHQ was measured in regions of the cell-surface membrane associated with or devoid of antibody-induced AChR clusters, respectively. AChR clusters were almost equally distributed between Lo and Ld domains, independently of receptor surface levels and agonist (carbamoylcholine and nicotine) or antagonist (α-bungarotoxin) binding. Cholesterol depletion diminished the cell membrane mean di-4-ANEPPDHQ GP and the number of AChR clusters associated with Ld membrane domains increased concomitantly. Depolymerization of the filamentous actin cytoskeleton by Latrunculin A had the opposite effect, with more AChR clusters associated with Lo domains. AChR internalized via small vesicles having lower GP and lower cholesterol content than the surface membrane. Upon cholesterol depletion, only 12% of the AChR-containing vesicles costained with the fluorescent cholesterol analog fPEG-cholesterol, i.e., AChR endocytosis was essentially dissociated from that of cholesterol. In conclusion, the distribution of AChR submicron-sized clusters at the cell membrane appears to be regulated by cholesterol content and cytoskeleton integrity.

Neurotrophins regulate cholinergic synaptic transmission in cultured rat sympathetic neurons through a p75-dependent mechanism

The sympathetic nervous system regulates many essential physiological systems, and its dysfunction is implicated in cardiovascular diseases. Mechanisms that control the strength of sympathetic output are therefore potential targets for the management of these disorders. Here we show that neurotrophins rapidly potentiate cholinergic transmission between cultured rat sympathetic neurons. We found that brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), acting at the p75 receptor, increased the amplitude of excitatory postsynaptic currents (EPSCs). We observed increased amplitude but not frequency of miniature synaptic currents after p75 activation, suggesting that p75 acts postsynaptically to modulate transmission at these synapses. This neurotrophic modulation enhances cholinergic EPSCs via sphingolipid signaling. Application of sphingolactone-24, an inhibitor of neutral sphingomyelinase, blocked the effect of BDNF, implicating a sphingolipid pathway. Furthermore, application of the p75-associated sphingolipid second messengers C(2)-ceramide and d-erythro-sphingosine restricted to the postsynaptic cell mimicked BDNF application. Postsynaptic blockade of ceramide production with fumonisin, a ceramide synthase inhibitor, blocked the effects of BDNF and d-erythro-sphingosine, implicating ceramide or ceramide phosphate as the active signal. Together these data suggest that neurotrophin signaling, which occurs in vivo via release from sympathetic neurons and target tissues such as the heart, acutely regulates the strength of the sympathetic postganglionic response to central cholinergic inputs. This pathway provides a potential mechanism for modulating the strength of sympathetic drive to target organs such as the heart and could play a role in the development of cardiovascular diseases.

Molecular mechanisms of acetylcholine receptor-lipid interactions: from model membranes to human biology

Lipids are potent modulators of the Torpedo nicotinic acetylcholine receptor. Lipids influence nicotinic receptor function by allosteric mechanisms, stabilizing varying proportions of pre-existing resting, open, desensitized, and uncoupled conformations. Recent structures reveal that lipids could alter function by modulating transmembrane α-helix/α-helix packing, which in turn could alter the conformation of the allosteric interface that links the agonist-binding and transmembrane pore domains-this interface is essential in the coupling of agonist binding to channel gating. We discuss potential mechanisms by which lipids stabilize different conformational states in the context of the hypothesis that lipid-nicotinic receptor interactions modulate receptor function at biological synapses.

Sphingomyelinase selectively reduces M1 muscarinic receptors in rat hippocampal membranes

Although there is evidence that nicotinic acetylcholine (Ach) receptors are influenced by ceramides, we do not currently know whether or not these sphingolipids can also regulate the muscarinic subtypes of Ach receptors. Using the whole-cell patch technique, we demonstrated that the effectiveness of the muscarinic receptor agonist pilocarpine, in enhancing spontaneous inhibitory postsynaptic currents in CA1 pyramidal cells, was completely abolished in hippocampal slices pre-exposed to the ceramide-generating enzyme sphingomyelinase (SMase). Western blot experiments, performed with biotinylated hippocampal membranes, showed that this electrophysiological defect possibly relies on the loss of M1 muscarinic Ach receptors at the cell surface. However, the effect appears to be relatively specific as the cell-surface expression of M4 muscarinic receptors was not found to be impacted by SMase treatment. Interestingly, we observed that G protein-coupled receptor kinases 2 and β-arrestin1/2 interactions with M1-immunoprecipitated proteins were substantially augmented in SMase-treated slices and that the reduction of cell-surface M1 muscarinic receptor expression generated was completely suppressed by the muscarinic antagonist atropine. Collectively, our data suggest that selective internalization of M1 muscarinic receptors can be accentuated in neurons subjected to high ceramide levels. The potential physiopathological implications of this finding are presented.

Disturbance in cerebral spinal fluid sphingolipid content is associated with memory impairment in subjects infected with the human immunodeficiency virus

Despite widespread use of antiretroviral therapies to control replication of the human immunodeficiency virus (HIV), dysfunctions of cognition that are collectively termed HIV-associated neurocognitive disorders (HAND) still occur in approximately 50% of those infected by the virus. Currently there is not a biomarker that can identify HIV-infected people who are at risk for the development of HAND. Previous studies have identified particular sphingolipid species that are dysregulated in HAND, but the neurocognitive correlates of these biochemical findings are not currently understood. To address this question, we compared cerebrospinal fluid (CSF) levels of sphingomyelin, ceramide, and sterol species with performance on standard neurological tests designed to assess the function of multiple cognitive and motor domains in HIV-infected subjects. We found that sphingomyelin:ceramide ratios for acyl chain lengths of C16:0, C18:0, C22:0, and C24:0 were associated with worse performance on several indices of memory. The most striking finding was for the acyl chain of C18:0 that consistently associated with performance on multiple tests of memory. These findings suggest that the sphingomyelin:ceramide ratio for C18:0 may be a reasonable surrogate marker for memory dysfunction in HIV-infected subjects.