Biological aspects of ceramide-enriched membrane domains

Using ultrasound and microbubble to enhance the effects of conventional cancer therapies in clinical settings

It has been demonstrated in preclinical research that the administration of microbubbles with ultrasound can augment the proapoptotic sphingolipid pathway and enhance chemotherapy or radiation therapy-induced vascular endothelial disruption resulting in enhanced tumor cell death. Specifically, ultrasound-stimulated microbubbles (USMB) can increase blood vessel permeability facilitating the release of therapeutic substances in the target area. USMB can also serve as a potential radiation enhancing therapy as USMB exposure increases tumor cell death significantly as observed in preclinical models. Clinical studies have found the combination of USMB and these existing cancer therapies to be safe and also to be associated with greater tumor responses. USMB-based treatment can be applicable in a clinical setting using either ultrasound imaging or magnetic resonance imaging (MRI) guidance for precise treatment. In the latter, the ultrasound device is integrated into the MRI system platform for sonication to facilitate microbubble stimulation. In this review, we concisely present findings related to USMB and existing cancer therapies (chemotherapy and radiation therapy) in clinical trial settings. The possible underlying mechanism involved in USMB-enhanced chemotherapy or radiotherapy enhancement is also discussed. Lastly, the study concludes with some limitations and an examination of the future direction of these combined therapies.

Indicaxanthin prevents eryptosis induced by cigarette smoke extract by interfering with active Fas-mediated signaling

A physiological mechanism of programmed cell death called eryptosis occurs in aged or damaged red blood cells (RBCs). Dysregulated eryptosis contributes to abnormal microcirculation and prothrombotic risk. Cigarette smoke extract (CSE) induces a p38 MAPK-initiated, Fas-mediated eryptosis, activating the death-inducing signaling complex (DISC). Indicaxanthin (Ind) from cactus pear fruits, is a bioavailable dietary phytochemical in humans and it is able to incorporate into RBCs enhancing their defense against numerous stimuli. This in vitro work shows that Ind, at concentrations that mimic plasma concentrations after a fruit meal, protects erythrocytes from CSE-induced eryptosis. CSE from commercial cigarettes was prepared in aqueous solution using an impinger air sampler and nicotine content was determined. RBCs were treated with CSE for 3 h in the absence or presence of increasing concentrations of Ind (from 1 to 5 μM). Cytofluorimetric measurements indicated that Ind reduced CSE-induced phosphatidylserine externalization and ceramide formation in a concentration-dependent manner. Confocal microscopy visualization and coimmunoprecipitation experiments revealed that Ind prevented both CSE-triggered Fas aggregation and FasL/FADD/caspase 8 recruitment in the membrane, indicating inhibition of DISC assembly. Ind inhibited the phosphorylation of p38 MAPK, caspase-8/caspase-3 cleavage, and caspase-3 activity induced by CSE. Finally, Ind reduced CSE-induced ATP depletion and restored aminophospholipid translocase activity impaired by CSE treatment. In conclusion, Ind concentrations comparable to nutritionally relevant plasma concentrations, can prevent Fas-mediated RBC death signaling induced by CSE, which suggests that dietary intake of cactus pear fruits may limit the deleterious effects of cigarette smoking.

Unveiling enigmatic essence of Sphingolipids: A promising avenue for glaucoma treatment

Treatment of glaucoma, the leading cause of irreversible blindness, remains challenging. The apoptotic loss of retinal ganglion cells (RGCs) in glaucoma is the pathological hallmark. Current treatments often remain suboptimal as they aim to halt RGC loss secondary to reduction of intraocular pressure. The pathophysiological targets for exploring direct neuroprotective approaches, therefore are highly relevant. Sphingolipids have emerged as significant target molecules as they are not only the structural components of various cell constituents, but they also serve as signaling molecules that regulate molecular pathways involved in cell survival and death. Investigations have shown that a critical balance among various sphingolipid species, particularly the ceramide and sphingosine-1-phosphate play a role in deciding the fate of the cell. In this review we briefly discuss the metabolic interconversion of sphingolipid species to get an insight into "sphingolipid rheostat", the dynamic balance among metabolites. Further we highlight the role of sphingolipids in the key pathophysiological mechanisms that lead to glaucomatous loss of RGCs. Lastly, we summarize the potential drug candidates that have been investigated for their neuroprotective effects in glaucoma via their effects on sphingolipid axis.

Sphingolipid Signaling and Complement Activation in Glioblastoma: A Promising Avenue for Therapeutic Intervention

Glioblastoma is the most common and aggressive type of malignant brain tumor with a poor prognosis due to the lack of effective treatment options. Therefore, new treatment options are required. Sphingolipids are essential components of the cell membrane, while complement components are integral to innate immunity, and both play a critical role in regulating glioblastoma survival signaling. This review focuses on recent studies investigating the functional roles of sphingolipid metabolism and complement activation signaling in glioblastoma. It also discusses how targeting these two systems together may emerge as a novel therapeutic approach.

Serotonin Signaling through Lipid Membranes

Serotonin (5-HT) is a vital modulatory neurotransmitter responsible for regulating most behaviors in the brain. An inefficient 5-HT synaptic function is often linked to various mental disorders. Primarily, membrane proteins controlling the expression and activity of 5-HT synthesis, storage, release, receptor activation, and inactivation are critical to 5-HT signaling in synaptic and extra-synaptic sites. Moreover, these signals represent information transmission across membranes. Although the lipid membrane environment is often viewed as fairly stable, emerging research suggests significant functional lipid-protein interactions with many synaptic 5-HT proteins. These protein-lipid interactions extend to almost all the primary lipid classes that form the plasma membrane. Collectively, these lipid classes and lipid-protein interactions affect 5-HT synaptic efficacy at the synapse. The highly dynamic lipid composition of synaptic membranes suggests that these lipids and their interactions with proteins may contribute to the plasticity of the 5-HT synapse. Therefore, this broader protein-lipid model of the 5-HT synapse necessitates a reconsideration of 5-HT's role in various associated mental disorders.

Sphingolipid-Induced Bone Regulation and Its Emerging Role in Dysfunction Due to Disease and Infection

The human skeleton is a metabolically active system that is constantly regenerating via the tightly regulated and highly coordinated processes of bone resorption and formation. Emerging evidence reveals fascinating new insights into the role of sphingolipids, including sphingomyelin, sphingosine, ceramide, and sphingosine-1-phosphate, in bone homeostasis. Sphingolipids are a major class of highly bioactive lipids able to activate distinct protein targets including, lipases, phosphatases, and kinases, thereby conferring distinct cellular functions beyond energy metabolism. Lipids are known to contribute to the progression of chronic inflammation, and notably, an increase in bone marrow adiposity parallel to elevated bone loss is observed in most pathological bone conditions, including aging, rheumatoid arthritis, osteoarthritis, and osteomyelitis. Of the numerous classes of lipids that form, sphingolipids are considered among the most deleterious. This review highlights the important primary role of sphingolipids in bone homeostasis and how dysregulation of these bioactive metabolites appears central to many chronic bone-related diseases. Further, their contribution to the invasion, virulence, and colonization of both viral and bacterial host cell infections is also discussed. Many unmet clinical needs remain, and data to date suggest the future use of sphingolipid-targeted therapy to regulate bone dysfunction due to a variety of diseases or infection are highly promising. However, deciphering the biochemical and molecular mechanisms of this diverse and extremely complex sphingolipidome, both in terms of bone health and disease, is considered the next frontier in the field.

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.

Neutral sphingomyelinase inhibition promotes local and network degeneration in vitro and in vivo

BACKGROUND

Cell-to-cell communication is vital for tissues to respond, adapt, and thrive in the prevailing milieu. Several mechanisms mediate intercellular signaling, including tunneling nanotubes, gap junctions, and extracellular vesicles (EV). Depending on local and systemic conditions, EVs may contain cargoes that promote survival, neuroprotection, or pathology. Our understanding of pathologic intercellular signaling has been bolstered by disease models using neurons derived from human pluripotent stems cells (hPSC).

METHODS

Here, we used hPSC-derived retinal ganglion cells (hRGC) and the mouse visual system to investigate the influence of modulating EV generation on intercellular trafficking and cell survival. We probed the impact of EV modulation on cell survival by decreasing the catabolism of sphingomyelin into ceramide through inhibition of neutral sphingomyelinase (nSMase), using GW4869. We assayed for cell survival in vitro by probing for annexin A5, phosphatidylserine, viable mitochondria, and mitochondrial reactive oxygen species. In vivo, we performed intraocular injections of GW4869 and measured RGC and superior colliculus neuron density and RGC anterograde axon transport.

RESULTS

Following twenty-four hours of dosing hRGCs with GW4869, we found that inhibition of nSMase decreased ceramide and enhanced GM1 ganglioside accumulation. This inhibition also reduced the density of small EVs, increased the density of large EVs, and enriched the pro-apoptotic protein, annexin A5. Reducing nSMase activity increased hRGC apoptosis initiation due to enhanced density and uptake of apoptotic particles, as identified by the annexin A5 binding phospholipid, phosphatidylserine. We assayed intercellular trafficking of mitochondria by developing a coculture system of GW4869-treated and naïve hRGCs. In treated cells, inhibition of nSMase reduced the number of viable mitochondria, while driving mitochondrial reactive oxygen species not only in treated, but also in naive hRGCs added in coculture. In mice, 20 days following a single intravitreal injection of GW4869, we found a significant loss of RGCs and their axonal recipient neurons in the superior colliculus. This followed a more dramatic reduction in anterograde RGC axon transport to the colliculus.

CONCLUSION

Overall, our data suggest that perturbing the physiologic catabolism of sphingomyelin by inhibiting nSMase reorganizes plasma membrane associated sphingolipids, alters the profile of neuron-generated EVs, and promotes neurodegeneration in vitro and in vivo by shifting the balance of pro-survival versus -degenerative EVs. Video Abstract.

Research progress of hyperthermia in tumor therapy by influencing metabolic reprogramming of tumor cells

Cellular metabolic reprogramming is an important feature of malignant tumors. Metabolic reprogramming causes changes in the levels or types of specific metabolites inside and outside the cell, which affects tumorigenesis and progression by influencing gene expression, the cellular state, and the tumor microenvironment. During tumorigenesis, a series of changes in the glucose metabolism, fatty acid metabolism, amino acid metabolism, and cholesterol metabolism of tumor cells occur, which are involved in the process of cellular carcinogenesis and constitute part of the underlying mechanisms of tumor formation. Hyperthermia, as one of the main therapeutic tools for malignant tumors, has obvious effects on tumor cell metabolism. In this paper, we will combine the latest research progress in the field of cellular metabolic reprogramming and focus on the current experimental research and clinical treatment of hyperthermia in cellular metabolic reprogramming to discuss the feasibility of cellular metabolic reprogramming-related mechanisms guiding hyperthermia in malignant tumor treatment, so as to provide more ideas for hyperthermia to treat malignant tumors through the direction of cellular metabolic reprogramming.

New insights into the roles of oligodendrocytes regulation in ischemic stroke recovery

Oligodendrocytes (OLs), the myelin-forming cells of the central nervous system, are integral to axonal integrity and function. Hypoxia-ischemia episodes can cause severe damage to these vulnerable cells through excitotoxicity, oxidative stress, inflammation, and mitochondrial dysfunction, leading to axonal dystrophy, neuronal dysfunction, and neurological impairments. OLs damage can result in demyelination and myelination disorders, severely impacting axonal function, structure, metabolism, and survival. Adult-onset stroke, periventricular leukomalacia, and post-stroke cognitive impairment primarily target OLs, making them a critical therapeutic target. Therapeutic strategies targeting OLs, myelin, and their receptors should be given more emphasis to attenuate ischemia injury and establish functional recovery after stroke. This review summarizes recent advances on the function of OLs in ischemic injury, as well as the present and emerging principles that serve as the foundation for protective strategies against OL deaths.

The Air-Liquid Interface Reorganizes Membrane Lipids and Enhances the Recruitment of Slc26a3 to Lipid-Rich Domains in Human Colonoid Monolayers

Cholesterol-rich membrane domains, also called lipid rafts (LRs), are specialized membrane domains that provide a platform for intracellular signal transduction. Membrane proteins often cluster in LRs that further aggregate into larger platform-like structures that are enriched in ceramides and are called ceramide-rich platforms (CRPs). The role of CRPs in the regulation of intestinal epithelial functions remains unknown. Down-regulated in adenoma (DRA) is an intestinal Cl-/HCO3- antiporter that is enriched in LRs. However, little is known regarding the mechanisms involved in the regulation of DRA activity. The air-liquid interface (ALI) was created by removing apical media for a specified number of days; from 12-14 days post-confluency, Caco-2/BBe cells or a colonoid monolayer were grown as submerged cultures. Confocal imaging was used to examine the dimensions of membrane microdomains that contained DRA. DRA expression and activity were enhanced in Caco-2/BBe cells and human colonoids using an ALI culture method. ALI causes an increase in acid sphingomyelinase (ASMase) activity, an enzyme responsible for enhancing ceramide content in the plasma membrane. ALI cultures expressed a larger number of DRA-containing platforms with dimensions >2 µm compared to cells grown as submerged cultures. ASMase inhibitor, desipramine, disrupted CRPs and reduced the ALI-induced increase in DRA expression in the apical membrane. Exposing normal human colonoid monolayers to ALI increased the ASMase activity and enhanced the differentiation of colonoids along with basal and forskolin-stimulated DRA activities. ALI increases DRA activity and expression by increasing ASMase activity and platform formation in Caco-2/BBe cells and by enhancing the differentiation of colonoids.

Rapid, sensitive, and high-throughput quantification of broad serological ceramides by using isotope dilution liquid chromatography-negative ion electrospray tandem mass spectrometry

Ceramides are important intermediates in the metabolism of sphingolipids. High-throughput liquid chromatography-mass spectrometry has been used extensively for monitoring the levels of serological ceramides, but is still limited by inadequate coverage or lack of sensitivity. Herein, a rapid, sensitive, and high-throughput isotope dilution liquid chromatography-negative ion electrospray tandem mass spectrometry (IDLC-nESI-MS/MS) method was developed and verified for accurate quantification of 41 ceramides, involving ceramides with C16-20 sphingosine, dihydro-ceramide, and dehydro-ceramide. This method was validated with excellent linearity (R² > 0.99) and good recovery in the range of 90-110%. Intra- and inter-day imprecision were below 5.57% and 7.83% respectively. The improved high-throughput quantitative method developed in this study may aid in the accurate characterization of ceramides for understanding ceramide biology and application in disease diagnosis.

A genome-wide CRISPR screen implicates plasma membrane asymmetry in exogenous C6-ceramide toxicity

The bioactive sphingolipid ceramide impacts diverse cellular processes (e.g. apoptosis and cell proliferation) through its effects on membrane dynamics and intracellular signaling pathways. The dysregulation of ceramide metabolism has been implicated in cancer evasion of apoptosis and targeting ceramide metabolism has potential therapeutic benefits as a strategy to kill cancer cells and slow tumor growth. However, the mechanisms of cancer cell resistance to ceramide-mediated cell death are vastly intertwined and incompletely understood. To shed light on this mystery, we performed a genome-wide CRISPR-Cas9 screen to systematically identify regulators of cancer resistance to the soluble short chain ceramide, C6 ceramide (C6-Cer). Our results reveal a complex landscape of genetic modifiers of C6-Cer toxicity, including genes associated with ceramide and sphingolipid metabolism, vesicular trafficking, and membrane biology. Furthermore, we find that loss of the phospholipid flippase subunit TMEM30A impairs the plasma membrane trafficking of its binding partner, the P4-type ATPase ATP11B, and depletion of TMEM30A or ATP11B disrupts plasma membrane asymmetry and promotes resistance to C6-Cer toxicity. Together, our findings provide a resource of genetic modifiers of C6-Cer toxicity and reveal an unexpected role of plasma membrane asymmetry in C6-Cer induced cell death.

Cigarette Smoke Extract Induces p38 MAPK-Initiated, Fas-Mediated Eryptosis

Eryptosis is a physiological mechanism for the clearance of senescent or damaged erythrocytes by phagocytes. Excessive eryptosis is stimulated under several pathologies and associated with endothelial injury and thrombosis. Cigarette smoke (CS) is an established risk factor for vascular diseases and cigarette smokers have high-levels of eryptotic erythrocytes. This study, for the first time, investigates the mechanism by which CS damages red blood cells (RBCs). CS extract (CSE) from commercial cigarettes was prepared and standardized for nicotine content. Cytofluorimetric analysis demonstrated that treatment of human RBCs with CSE caused dose-dependent, phosphatidylserine externalization and cell shrinkage, hallmarks of apoptotic death. CSE did not affect cellular levels of Ca²⁺, reactive oxygen species (ROS) or glutathione (GSH). Immununoprecipitation and immunoblotting revealed the assembly of the death-inducing signaling complex (DISC) and oligomerization of Fas receptor as well as cleaved caspase-8 and caspase-3 within 6 h from the treatment. At the same time-interval, CSE elicited neutral sphyngomielinase (nSMase) activity-dependent ceramide formation and phosphorylation of p38 MAPK. Through specific inhibitors' nSMase, caspase-8 or p38 MAPK activities, we demonstrated that p38 MAPK activation is required for caspase-8-mediated eryptosis and that ceramide generation is initiator caspase-dependent. Finally, ex vivo analysis detected phosphorylated p38 MAPK (p-p38) and Fas-associated signaling complex in erythrocytes from cigarette smokers. In conclusion, our study demonstrates that CSE exposure induces in erythrocytes an extrinsic apoptotic pathway involving p38 MAPK-initiated DISC formation followed by activation of caspase-8/caspase-3 via ceramide formation.

Involvement of Ceramide Signalling in Radiation-Induced Tumour Vascular Effects and Vascular-Targeted Therapy

Sphingolipids are well-recognized critical components in several biological processes. Ceramides constitute a class of sphingolipid metabolites that are involved in important signal transduction pathways that play key roles in determining the fate of cells to survive or die. Ceramide accumulated in cells causes apoptosis; however, ceramide metabolized to sphingosine promotes cell survival and angiogenesis. Studies suggest that vascular-targeted therapies increase endothelial cell ceramide resulting in apoptosis that leads to tumour cure. Specifically, ultrasound-stimulated microbubbles (USMB) used as vascular disrupting agents can perturb endothelial cells, eliciting acid sphingomyelinase (ASMase) activation accompanied by ceramide release. This phenomenon results in endothelial cell death and vascular collapse and is synergistic with other antitumour treatments such as radiation. In contrast, blocking the generation of ceramide using multiple approaches, including the conversion of ceramide to sphingosine-1-phosphate (S1P), abrogates this process. The ceramide-based cell survival "rheostat" between these opposing signalling metabolites is essential in the mechanotransductive vascular targeting following USMB treatment. In this review, we aim to summarize the past and latest findings on ceramide-based vascular-targeted strategies, including novel mechanotransductive methodologies.

An Overview of Cell Membrane Perforation and Resealing Mechanisms for Localized Drug Delivery

Localized and reversible plasma membrane disruption is a promising technique employed for the targeted deposition of exogenous therapeutic compounds for the treatment of disease. Indeed, the plasma membrane represents a significant barrier to successful delivery, and various physical methods using light, sound, and electrical energy have been developed to generate cell membrane perforations to circumvent this issue. To restore homeostasis and preserve viability, localized cellular repair mechanisms are subsequently triggered to initiate a rapid restoration of plasma membrane integrity. Here, we summarize the known emergency membrane repair responses, detailing the salient membrane sealing proteins as well as the underlying cytoskeletal remodeling that follows the physical induction of a localized plasma membrane pore, and we present an overview of potential modulation strategies that may improve targeted drug delivery approaches.

Mapping Sphingolipid Metabolism Pathways during Phagosomal Maturation

Phagocytosis is an important physiological process, which, in higher organisms, is a means of fighting infections and clearing cellular debris. During phagocytosis, detrimental foreign particles (e.g. pathogens and apoptotic cells) are engulfed by phagocytes (e.g. macrophages), enclosed in membrane-bound vesicles called phagosomes, and transported to the lysosome for eventual detoxification. During this well-choreographed process, the nascent phagosome (also called early phagosome, EP) undergoes a series of spatiotemporally regulated changes in its protein and lipid composition and matures into a late phagosome (LP), which subsequently fuses with the lysosomal membrane to form the phagolysosome. While several elegant proteomic studies have identified the role of unique proteins during phagosomal maturation, the corresponding lipidomic studies are sparse. Recently, we reported a comparative lipidomic analysis between EPs and LPs and showed that ceramides are enriched on the LPs. Further, we found that this ceramide accumulation on LPs was orchestrated by ceramide synthase 2, inhibition of which hampers phagosomal maturation. Following up on this study, here, using biochemical assays, we first show that the increased ceramidase activity on EPs also significantly contributes to the accumulation of ceramides on LPs. Next, leveraging lipidomics, we show that de novo ceramide synthesis does not significantly contribute to the ceramide accumulation on LPs, while concomitant to increased ceramides, glucosylceramides are substantially elevated on LPs. We validate this interesting finding using biochemical assays and show that LPs indeed have heightened glucosylceramide synthase activity. Taken together, our studies provide interesting insights and possible new roles of sphingolipid metabolism during phagosomal maturation.

Molecular and mesoscopic geometries in autophagosome generation. A review

Autophagy is an essential process in cell self-repair and survival. The centre of the autophagic event is the generation of the so-called autophagosome (AP), a vesicle surrounded by a double membrane (two bilayers). The AP delivers its cargo to a lysosome, for degradation and re-use of the hydrolysis products as new building blocks. AP formation is a very complex event, requiring dozens of specific proteins, and involving numerous instances of membrane biogenesis and architecture, including membrane fusion and fission. Many stages of AP generation can be rationalised in terms of curvature, both the molecular geometry of lipids interpreted in terms of 'intrinsic curvature', and the overall mesoscopic curvature of the whole membrane, as observed with microscopy techniques. The present contribution intends to bring together the worlds of biophysics and cell biology of autophagy, in the hope that the resulting cross-pollination will generate abundant fruit.

The Manifold Roles of Sphingolipids in Viral Infections

Sphingolipids are essential components of eukaryotic cells. In this review, we want to exemplarily illustrate what is known about the interactions of sphingolipids with various viruses at different steps of their replication cycles. This includes structural interactions during entry at the plasma membrane or endosomal membranes, early interactions leading to sphingolipid-mediated signal transduction, interactions with internal membranes and lipids during replication, and interactions during virus assembly and budding. Targeted interventions in sphingolipid metabolism - as far as they can be tolerated by cells and organisms - may open novel possibilities to support antiviral therapies. Human immunodeficiency virus type 1 (HIV-1) infections have intensively been studied, but for other viral infections, such as influenza A virus (IAV), measles virus (MV), hepatitis C virus (HCV), dengue virus, Ebola virus, and severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2), investigations are still in their beginnings. As many inhibitors of sphingolipid metabolism are already in clinical use against other diseases, repurposing studies for applications in some viral infections appear to be a promising approach.

Sphingomyelinase-Mediated Multitimescale Clustering of Ganglioside GM1 in Heterogeneous Lipid Membranes

Several signaling processes in the plasma membrane are intensified by ceramides that are formed by sphingomyelinase-mediated hydrolysis of sphingomyelin. These ceramides trigger clustering of signaling-related biomolecules, but how they concentrate such biomolecules remains unclear. Here, the spatiotemporal localization of ganglioside GM1, a glycolipid receptor involved in signaling, during sphingomyelinase-mediated hydrolysis is described. Real-time visualization of the dynamic remodeling of the heterogeneous lipid membrane that occurs due to sphingomyelinase action is used to examine GM1 clustering, and unexpectedly, it is found that it is more complex than previously thought. Specifically, lipid membranes generate two distinct types of condensed GM1: 1) rapidly formed but short-lived GM1 clusters that are formed in ceramide-rich domains nucleated from the liquid-disordered phase; and 2) late-onset yet long-lasting, high-density GM1 clusters that are formed in the liquid-ordered phase. These findings suggest that multiple pathways exist in a plasma membrane to synergistically facilitate the rapid amplification and persistence of signals.

The roles of lipids in SARS-CoV-2 viral replication and the host immune response

The significant morbidity and mortality associated with severe acute respiratory syndrome coronavirus 2 infection has underscored the need for novel antiviral strategies. Lipids play essential roles in the viral life cycle. The lipid composition of cell membranes can influence viral entry by mediating fusion or affecting receptor conformation. Upon infection, viruses can reprogram cellular metabolism to remodel lipid membranes and fuel the production of new virions. Furthermore, several classes of lipid mediators, including eicosanoids and sphingolipids, can regulate the host immune response to viral infection. Here, we summarize the existing literature on the mechanisms through which these lipid mediators may regulate viral burden in COVID-19. Furthermore, we define the gaps in knowledge and identify the core areas in which lipids offer therapeutic promise for severe acute respiratory syndrome coronavirus 2.

Acid sphingomyelinase promotes SGK1-dependent vascular calcification

In chronic kidney disease (CKD), hyperphosphatemia is a key factor promoting medial vascular calcification, a common complication associated with cardiovascular events and high mortality. Vascular calcification involves osteo-/chondrogenic transdifferentiation of vascular smooth muscle cells (VSMCs), but the complex signaling events inducing pro-calcific pathways are incompletely understood. The present study investigated the role of acid sphingomyelinase (ASM)/ceramide as regulator of VSMC calcification. In vitro, both, bacterial sphingomyelinase and phosphate increased ceramide levels in VSMCs. Bacterial sphingomyelinase as well as ceramide supplementation stimulated osteo-/chondrogenic transdifferentiation during control and high phosphate conditions and augmented phosphate-induced calcification of VSMCs. Silencing of serum- and glucocorticoid-inducible kinase 1 (SGK1) blunted the pro-calcific effects of bacterial sphingomyelinase or ceramide. Asm deficiency blunted vascular calcification in a cholecalciferol-overload mouse model and ex vivo isolated-perfused arteries. In addition, Asm deficiency suppressed phosphate-induced osteo-/chondrogenic signaling and calcification of cultured VSMCs. Treatment with the functional ASM inhibitors amitriptyline or fendiline strongly blunted pro-calcific signaling pathways in vitro and in vivo. In conclusion, ASM/ceramide is a critical upstream regulator of vascular calcification, at least partly, through SGK1-dependent signaling. Thus, ASM inhibition by repurposing functional ASM inhibitors to reduce the progression of vascular calcification during CKD warrants further study.

New Molecular Targets for Antidepressant Drugs

Major depressive disorder (MDD) is a common and severe mental disorder that is usually recurrent and has a high risk of suicide. This disorder manifests not only with psychological symptoms but also multiple changes throughout the body, including increased risks of obesity, diabetes, and cardiovascular disease. Peripheral markers of oxidative stress and inflammation are elevated. MDD is therefore best described as a multisystem whole-body disease. Pharmacological treatment with antidepressants usually requires several weeks before the desired effects manifest. Previous theories of depression, such as the monoamine or neurogenesis hypotheses, do not explain these characteristics well. In recent years, new mechanisms of action have been discovered for long-standing antidepressants that also shed new light on depression, including the sphingolipid system and the receptor for brain-derived neurotrophic factor (BDNF).

Regulation of sphingolipid synthesis by the G1/S transcription factor Swi4

SBF (Swi4/Swi6 Binding Factor) complex is a crucial regulator of G1/S transition in Saccharomyces cerevisiae. Here, we show that SBF complex is required for myriocin resistance, an inhibitor of sphingolipid synthesis. This phenotype was not shared with MBF complex mutants nor with deletion of the Swi4p downstream targets, CLN1/CLN2. Based on data mining results, we selected putative Swi4p targets related to sphingolipid metabolism and studied their gene transcription as well as metabolite levels during progression of the cell cycle. Genes which encode key enzymes for the synthesis of long chain bases (LCBs) and ceramides were periodically transcribed during the mitotic cell cycle, having a peak at G1/S, and required SWI4 for full transcription at this stage. In addition, HPLC-MS/MS data indicated that swi4Δ cells have decreased levels of sphingolipids during progression of the cell cycle, particularly, dihydrosphingosine (DHS), C24-phytoceramides and C24-inositolphosphoryl ceramide (IPC) while it had increased levels of mannosylinositol phosphorylceramide (MIPC). Furthermore, we demonstrated that both inhibition of de novo sphingolipid synthesis by myriocin or SWI4 deletion caused partial arrest at the G2/M phase. Importantly, our lipidomic data demonstrated that the sphingolipid profile of WT cells treated with myriocin resembled that of swi4Δ cells, with lower levels of DHS, IPC and higher levels of MIPC. Taken together, these results show that SBF complex plays an essential role in the regulation of sphingolipid homeostasis, which reflects in the correct progression through the G2/M phase of the cell cycle.

Sphingolipids: Effectors and Achilles Heals in Viral Infections?

As viruses are obligatory intracellular parasites, any step during their life cycle strictly depends on successful interaction with their particular host cells. In particular, their interaction with cellular membranes is of crucial importance for most steps in the viral replication cycle. Such interactions are initiated by uptake of viral particles and subsequent trafficking to intracellular compartments to access their replication compartments which provide a spatially confined environment concentrating viral and cellular components, and subsequently, employ cellular membranes for assembly and exit of viral progeny. The ability of viruses to actively modulate lipid composition such as sphingolipids (SLs) is essential for successful completion of the viral life cycle. In addition to their structural and biophysical properties of cellular membranes, some sphingolipid (SL) species are bioactive and as such, take part in cellular signaling processes involved in regulating viral replication. It is especially due to the progress made in tools to study accumulation and dynamics of SLs, which visualize their compartmentalization and identify interaction partners at a cellular level, as well as the availability of genetic knockout systems, that the role of particular SL species in the viral replication process can be analyzed and, most importantly, be explored as targets for therapeutic intervention.

Sphingolipids as critical players in retinal physiology and pathology

Sphingolipids have emerged as bioactive lipids involved in the regulation of many physiological and pathological processes. In the retina, they have been established to participate in numerous processes, such as neuronal survival and death, proliferation and migration of neuronal and vascular cells, inflammation, and neovascularization. Dysregulation of sphingolipids is therefore crucial in the onset and progression of retinal diseases. This review examines the involvement of sphingolipids in retinal physiology and diseases. Ceramide (Cer) has emerged as a common mediator of inflammation and death of neuronal and retinal pigment epithelium cells in animal models of retinopathies such as glaucoma, age-related macular degeneration (AMD), and retinitis pigmentosa. Sphingosine-1-phosphate (S1P) has opposite roles, preventing photoreceptor and ganglion cell degeneration but also promoting inflammation, fibrosis, and neovascularization in AMD, glaucoma, and pro-fibrotic disorders. Alterations in Cer, S1P, and ceramide 1-phosphate may also contribute to uveitis. Notably, use of inhibitors that either prevent Cer increase or modulate S1P signaling, such as Myriocin, desipramine, and Fingolimod (FTY720), preserves neuronal viability and retinal function. These findings underscore the relevance of alterations in the sphingolipid metabolic network in the etiology of multiple retinopathies and highlight the potential of modulating their metabolism for the design of novel therapeutic approaches.

Cross-talks among GBA mutations, glucocerebrosidase, and α-synuclein in GBA-associated Parkinson's disease and their targeted therapeutic approaches: a comprehensive review

Current therapies for Parkinson's disease (PD) are palliative, of which the levodopa/carbidopa therapy remains the primary choice but is unable to modulate the progression of neurodegeneration. Due to the complication of such a multifactorial disorder and significant limitations of the therapy, numerous genetic approaches have been proved effective in finding out genes and mechanisms implicated in this disease. Following the observation of a higher frequency of PD in Gaucher's disease (GD), a lysosomal storage condition, mutations of glycosylceramidase beta (GBA) encoding glucocerebrosidase (GCase) have been shown to be involved and have been explored in the context of PD. GBA mutations are the most common genetic risk factor of PD. Various studies have revealed the relationships between PD and GBA gene mutations, facilitating a better understanding of this disorder. Various hypotheses delineate that the pathological mutations of GBA minimize the enzymatic activity of GCase, which affects the proliferation and clearance of α-synuclein; this affects the lysosomal homeostasis, exacerbating the endoplasmic reticulum stress or encouraging the mitochondrial dysfunction. Identification of the pathological mechanisms underlying the GBA-associated parkinsonism (GBA + PD) advances our understanding of PD. This review based on current literature aims to elucidate various genetic and clinical characteristics correlated with GBA mutations and to identify the numerous pathological processes underlying GBA + PD. We also delineate the therapeutic strategies to interfere with the mutant GCase function for further improvement of the related α-synuclein-GCase crosstalks. Moreover, the various therapeutic approaches such as gene therapy, chaperone proteins, and histone deacetylase inhibitors for the treatment of GBA + PD are discussed.

Cross-talks among GBA mutations, glucocerebrosidase, and α-synuclein in GBA-associated Parkinson’s disease and their targeted therapeutic approaches: a comprehensive review

Current therapies for Parkinson’s disease (PD) are palliative, of which the levodopa/carbidopa therapy remains the primary choice but is unable to modulate the progression of neurodegeneration. Due to the complication of such a multifactorial disorder and significant limitations of the therapy, numerous genetic approaches have been proved effective in finding out genes and mechanisms implicated in this disease. Following the observation of a higher frequency of PD in Gaucher’s disease (GD), a lysosomal storage condition, mutations of glycosylceramidase beta (GBA) encoding glucocerebrosidase (GCase) have been shown to be involved and have been explored in the context of PD. GBA mutations are the most common genetic risk factor of PD. Various studies have revealed the relationships between PD and GBA gene mutations, facilitating a better understanding of this disorder. Various hypotheses delineate that the pathological mutations of GBA minimize the enzymatic activity of GCase, which affects the proliferation and clearance of α-synuclein; this affects the lysosomal homeostasis, exacerbating the endoplasmic reticulum stress or encouraging the mitochondrial dysfunction. Identification of the pathological mechanisms underlying the GBA-associated parkinsonism (GBA + PD) advances our understanding of PD. This review based on current literature aims to elucidate various genetic and clinical characteristics correlated with GBA mutations and to identify the numerous pathological processes underlying GBA + PD. We also delineate the therapeutic strategies to interfere with the mutant GCase function for further improvement of the related α-synuclein–GCase crosstalks. Moreover, the various therapeutic approaches such as gene therapy, chaperone proteins, and histone deacetylase inhibitors for the treatment of GBA + PD are discussed.

The Manifold Roles of Sphingolipids in Viral Infections

Sphingolipids are essential components of eukaryotic cells. In this review, we want to exemplarily illustrate what is known about the interactions of sphingolipids with various viruses at different steps of their replication cycles. This includes structural interactions during entry at the plasma membrane or endosomal membranes, early interactions leading to sphingolipid-mediated signal transduction, interactions with internal membranes and lipids during replication, and interactions during virus assembly and budding. Targeted interventions in sphingolipid metabolism - as far as they can be tolerated by cells and organisms - may open novel possibilities to support antiviral therapies. Human immunodeficiency virus type 1 (HIV-1) infections have intensively been studied, but for other viral infections, such as influenza A virus (IAV), measles virus (MV), hepatitis C virus (HCV), dengue virus, Ebola virus, and severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2), investigations are still in their beginnings. As many inhibitors of sphingolipid metabolism are already in clinical use against other diseases, repurposing studies for applications in some viral infections appear to be a promising approach.

Nanoscale imaging of bacterial infections by sphingolipid expansion microscopy

Expansion microscopy (ExM) enables super-resolution imaging of proteins and nucleic acids on conventional microscopes. However, imaging of details of the organization of lipid bilayers by light microscopy remains challenging. We introduce an unnatural short-chain azide- and amino-modified sphingolipid ceramide, which upon incorporation into membranes can be labeled by click chemistry and linked into hydrogels, followed by 4× to 10× expansion. Confocal and structured illumination microscopy (SIM) enable imaging of sphingolipids and their interactions with proteins in the plasma membrane and membrane of intracellular organelles with a spatial resolution of 10–20 nm. As our functionalized sphingolipids accumulate efficiently in pathogens, we use sphingolipid ExM to investigate bacterial infections of human HeLa229 cells by Neisseria gonorrhoeae, Chlamydia trachomatis and Simkania negevensis with a resolution so far only provided by electron microscopy. In particular, sphingolipid ExM allows us to visualize the inner and outer membrane of intracellular bacteria and determine their distance to 27.6 ± 7.7 nm.

Imaging of lipid bilayers using light microscopy is challenging. Here the authors label cells using a short chain click-compatible ceramide to visualize mammalian and bacterial membranes with expansion microscopy.

Acid Sphingomyelinase Impacts Canonical Transient Receptor Potential Channels 6 (TRPC6) Activity in Primary Neuronal Systems

: The acid sphingomyelinase (ASM)/ceramide system exhibits a crucial role in the pathology of major depressive disorder (MDD). ASM hydrolyzes the abundant membrane lipid sphingomyelin to ceramide that regulates the clustering of membrane proteins via microdomain and lipid raft organization. Several commonly used antidepressants, such as fluoxetine, rely on the functional inhibition of ASM in terms of their antidepressive pharmacological effects. Transient receptor potential canonical 6 (TRPC6) ion channels are located in the plasma membrane of neurons and serve as receptors for hyperforin, a phytochemical constituent of the antidepressive herbal remedy St. John's wort. TRPC6 channels are involved in the regulation of neuronal plasticity, which likely contributes to their antidepressant effect. In this work, we investigated the impact of reduced ASM activity on the TRPC6 function in neurons. A lipidomic analysis of cortical brain tissue of ASM deficient mice revealed a decrease in ceramide/sphingomyelin molar ratio and an increase in sphingosine. In neurons with ASM deletion, hyperforin-mediated Ca2+-influx via TRPC6 was decreased. Consequently, downstream activation of nuclear phospho-cAMP response element-binding protein (pCREB) was changed, a transcriptional factor involved in neuronal plasticity. Our study underlines the importance of balanced ASM activity, as well as sphingolipidome composition for optimal TRPC6 function. A better understanding of the interaction of the ASM/ceramide and TRPC6 systems could help to draw conclusions about the pathology of MDD.

Lipid rafts and neurodegeneration: structural and functional roles in physiologic aging and neurodegenerative diseases

Lipid rafts are small, dynamic membrane areas characterized by the clustering of selected membrane lipids as the result of the spontaneous separation of glycolipids, sphingolipids, and cholesterol in a liquid-ordered phase. The exact dynamics underlying phase separation of membrane lipids in the complex biological membranes are still not fully understood. Nevertheless, alterations in the membrane lipid composition affect the lateral organization of molecules belonging to lipid rafts. Neural lipid rafts are found in brain cells, including neurons, astrocytes, and microglia, and are characterized by a high enrichment of specific lipids depending on the cell type. These lipid rafts seem to organize and determine the function of multiprotein complexes involved in several aspects of signal transduction, thus regulating the homeostasis of the brain. The progressive decline of brain performance along with physiological aging is at least in part associated with alterations in the composition and structure of neural lipid rafts. In addition, neurodegenerative conditions, such as lysosomal storage disorders, multiple sclerosis, and Parkinson's, Huntington's, and Alzheimer's diseases, are frequently characterized by dysregulated lipid metabolism, which in turn affects the structure of lipid rafts. Several events underlying the pathogenesis of these diseases appear to depend on the altered composition of lipid rafts. Thus, the structure and function of lipid rafts play a central role in the pathogenesis of many common neurodegenerative diseases.jlr;61/5/636/F1F1f1.

The role of ceramides in metabolic disorders: when size and localization matters

Ceramide accumulation is a hallmark in the manifestation of numerous obesity-related diseases, such as type 2 diabetes mellitus and atherosclerosis. Until the early 2000s, ceramides were viewed as a homogenous class of sphingolipids. However, it has now become clear that ceramides exert fundamentally different effects depending on the specific fatty acyl chain lengths, which are integrated into ceramides by a group of enzymes known as dihydroceramide synthases. In addition, alterations in ceramide synthesis, trafficking and metabolism in specific cellular compartments exert distinct consequences on metabolic homeostasis. Here, we examine the emerging concept of how the intracellular localization of ceramides with distinct acyl chain lengths can regulate glucose metabolism, thus emphasizing their potential as targets in the development of novel and specific therapies for obesity and obesity-associated diseases.

Elevated plasma ceramide levels in post-menopausal women: a cross-sectional study

Circulating ceramide levels are abnormally elevated in age-dependent pathologies such as cardiovascular diseases, obesity and Alzheimer's disease. Nevertheless, the potential impact of age on plasma ceramide levels has not yet been systematically examined. In the present study, we quantified a focused panel of plasma ceramides and dihydroceramides in a cohort of 164 subjects (84 women) 19 to 80 years of age. After adjusting for potential confounders, multivariable linear regression analysis revealed a positive association between age and ceramide (d18:1/24:0) (β (SE) = 5.67 (2.38); p = .0198) and ceramide (d18:1/24:1) (β (SE) = 2.88 (.61); p < .0001) in women, and between age and ceramide (d18:1/24:1) in men (β (SE) = 1.86 (.77); p = .0179). In women of all ages, but not men, plasma ceramide (d18:1/24:1) was negatively correlated with plasma estradiol (r = -0.294; p = .007). Finally, in vitro experiments in human cancer cells expressing estrogen receptors showed that incubation with estradiol (10 nM, 24 h) significantly decreased ceramide accumulation. Together, the results suggest that aging is associated with an increase in circulating ceramide levels, which in post-menopausal women is at least partially associated with lower estradiol levels.

Targeting the Sphingolipid System as a Therapeutic Direction for Glioblastoma

Glioblastoma (GBM) is the most commonly diagnosed malignant brain tumor in adults. The prognosis for patients with GBM remains poor and largely unchanged over the last 30 years, due to the limitations of existing therapies. Thus, new therapeutic approaches are desperately required. Sphingolipids are highly enriched in the brain, forming the structural components of cell membranes, and are major lipid constituents of the myelin sheaths of nerve axons, as well as playing critical roles in cell signaling. Indeed, a number of sphingolipids elicit a variety of cellular responses involved in the development and progression of GBM. Here, we discuss the role of sphingolipids in the pathobiology of GBM, and how targeting sphingolipid metabolism has emerged as a promising approach for the treatment of GBM.

Glycosphingolipids and Infection. Potential New Therapeutic Avenues

Glycosphingolipids (GSLs), the main topic of this review, are a subclass of sphingolipids. With their glycans exposed to the extracellular space, glycosphingolipids are ubiquitous components of the plasma membrane of cells. GSLs are implicated in a variety of biological processes including specific infections. Several pathogens use GSLs at the surface of host cells as binding receptors. In addition, lipid-rafts in the plasma membrane of host cells may act as platform for signaling the presence of pathogens. Relatively common in man are inherited deficiencies in lysosomal glycosidases involved in the turnover of GSLs. The associated storage disorders (glycosphingolipidoses) show lysosomal accumulation of substrate(s) of the deficient enzyme. In recent years compounds have been identified that allow modulation of GSLs levels in cells. Some of these agents are well tolerated and already used to treat lysosomal glycosphingolipidoses. This review summarizes present knowledge on the role of GSLs in infection and subsequent immune response. It concludes with the thought to apply glycosphingolipid-lowering agents to prevent and/or combat infections.

The role of ceramide in regulating endoplasmic reticulum function

Sphingolipids (SLs) are an important class of membrane lipids containing a long chain sphingoid base backbone. SL synthesis is compartmentalized between two major cell organelles, the endoplasmic reticulum (ER) and the Golgi apparatus. The initial steps of sphingolipid synthesis take place in the ER, where the simplest SL, ceramide, is synthesized. Although ceramide is a critical membrane component, an imbalance of ceramide levels can have significant deleterious effects on cell properties leading to events such as apoptosis. For this reason and others, ER ceramide levels must be tightly regulated. Here, we describe the biological and biophysical properties of ceramide and discuss how this might impact the ER membrane. This article is part of a special issue entitled: ER Platforms for Membrane Lipid Dynamics.

Solving the secretory acid sphingomyelinase puzzle: Insights from lysosome-mediated parasite invasion and plasma membrane repair

Acid sphingomyelinase (ASM) is a lysosomal enzyme that cleaves the phosphorylcholine head group of sphingomyelin, generating ceramide. Recessive mutations in SMPD1, the gene encoding ASM, cause Niemann-Pick Disease Types A and B. These disorders are attributed not only to lipid accumulation inside lysosomes but also to changes on the outer leaflet of the plasma membrane, highlighting an extracellular role for ASM. Secretion of ASM occurs under physiological conditions, and earlier studies proposed two forms of the enzyme, one resident in lysosomes and another form that would be diverted to the secretory pathway. Such differential intracellular trafficking has been difficult to explain because there is only one SMPD1 transcript that generates an active enzyme, found primarily inside lysosomes. Unexpectedly, studies of cell invasion by the protozoan parasite Trypanosoma cruzi revealed that conventional lysosomes can fuse with the plasma membrane in response to elevations in intracellular Ca²⁺ , releasing their contents extracellularly. ASM exocytosed from lysosomes remodels the outer leaflet of the plasma membrane, promoting parasite invasion and wound repair. Here, we discuss the possibility that ASM release during lysosomal exocytosis, in response to various forms of stress, may represent a major source of the secretory form of this enzyme.

Agonists that stimulate secretion promote the recruitment of CFTR into membrane lipid microdomains

The cystic fibrosis transmembrane conductance regulator (CFTR) is a tightly regulated anion channel that mediates secretion by epithelia and is mutated in the disease cystic fibrosis. CFTR forms macromolecular complexes with many proteins; however, little is known regarding its associations with membrane lipids or the regulation of its distribution and mobility at the cell surface. We report here that secretagogues (agonists that stimulate secretion) such as the peptide hormone vasoactive intestinal peptide (VIP) and muscarinic agonist carbachol increase CFTR aggregation into cholesterol-dependent clusters, reduce CFTR lateral mobility within and between membrane microdomains, and trigger the fusion of clusters into large (3.0 µm2) ceramide-rich platforms. CFTR clusters are closely associated with motile cilia and with the enzyme acid sphingomyelinase (ASMase) that is constitutively bound on the cell surface. Platform induction is prevented by pretreating cells with cholesterol oxidase to disrupt lipid rafts or by exposure to the ASMase functional inhibitor amitriptyline or the membrane-impermeant reducing agent 2-mercaptoethanesulfonate. Platforms are reversible, and their induction does not lead to an increase in apoptosis; however, blocking platform formation does prevent the increase in CFTR surface expression that normally occurs during VIP stimulation. These results demonstrate that CFTR is colocalized with motile cilia and reveal surprisingly robust regulation of CFTR distribution and lateral mobility, most likely through autocrine redox activation of extracellular ASMase. Formation of ceramide-rich platforms containing CFTR enhances transepithelial secretion and likely has other functions related to inflammation and mucosal immunity.

The function of sphingomyelinases in mycobacterial infections

Tuberculosis (TB), caused by Mycobacterium tuberculosis, is one of the deadliest and most important infectious diseases worldwide. The sphingomyelinase/ceramide system, which has been shown several times to be a crucial factor in the internalization, processing and killing of diverse pathogens, also modulates the pro-inflammatory response and the state of mycobacteria in macrophages. Both acid and neutral sphingomyelinases are important in this activity. However, studies of the role of sphingomyelinases in TB are still at an early stage.

Acid sphingomyelinase mediates the noise-induced liver disorder in mice

Noise-induced structural and functional disorder of the liver has been realized, but the underlying mechanism remains to be characterized, which has limited the introduction of precautious measures. Over-activation of acid sphingomyelinase (ASM)/ceramide (Cer) pathway takes centre stage in hepatocyte injury entailed by various stimulus. We aimed to investigate whether it mediated the noise elicited liver disorder on infrastructure, lipid metabolism, apoptosis, and oxidative stress. Mice were exposed to broad band noise (20-20k Hz, 90-110 dB) for 1, 3, 5 or 7 days by 3 hr/d. Doxepin hydrochloride (DOX), an ASM inhibitor was given by 5 mg/kg/d gavage. We showed that 5 or 7 days intense, broad band noise exposure caused significant infrastructure derangement and lipid droplets storage in hepatocytes. The content of cholesterol, free fatty acids or triglyceride was increased significantly in liver tissue upon noise stimulation. Moreover, the noise promoted apoptosis and superoxide generation in hepatocytes significantly, enhancing activity of aspartate aminotransferase (AST) or alanine amino transferase (ALT) in serum. Acid sphingomyelinase activity and Cer generation in liver tissue were elevated by noise exposure, which was normalized with DOX administrated. Accordingly, DOX alleviated steatosis, apoptosis, oxidative stress and enzymatic change in hepatocytes or serum of noise exposed mice substantially. In summary, our results suggest the ASM/Cer pathway contributes to the broad band noise elicited liver damage in mice.

Anoctamin-6 regulates ADAM sheddase function

ADAM17, a prominent member of the "Disintegrin and Metalloproteinase" (ADAM) family, controls vital cellular functions through cleavage of transmembrane substrates including TGF-alpha, Amphiregulin (AREG) and TNF-Receptor 1 (TNFR1). We recently presented evidence that surface exposure of phosphatidylserine (PS) is pivotal for ADAM17 to exert sheddase activity. Anoctamin-6 (ANO6) has Ca2+-dependent phospholipid scramblase activity and it followed that the functions of ANO6 and ADAM17 might be linked. We report that overexpression of ANO6 in HEK293T cells led to increased Ca2+-mediated PS-exposure that was indeed accompanied by enhanced release of AREG and TGF-alpha. The effect was not observed when cells were treated with the PKC-dependent ADAM17 activator PMA. Transformation of cells with a constitutively active ANO6 mutant led to spontaneous PS-exposure and to the release of ADAM17-substrates in the absence of any stimuli. Inhibitor experiments indicated that ANO6-mediated enhancement of substrate cleavage simultaneously broadened the spectrum of participating metalloproteinases. In complementary experiments, siRNA-mediated downregulation of ANO6 was shown to decrease ionophore-mediated release of TNFR1 in human umbilical vein endothelial cells (HUVECs). We conclude that ANO6, by virtue of its scramblase activity, may play a role as an important regulator of the ADAM-network in the plasma membrane.

Clearly Detectable, Kinetically Restricted Solid-Solid Phase Transition in cis-Ceramide Monolayers

Sphingosine [(2 S,3 R,4 E)-2-amino-4-octadecene-1,3-diol] is the most common sphingoid base in mammals. Ceramides are N-acyl sphingosines. Numerous small variations on this canonical structure are known, including the 1-deoxy, the 4,5-dihydro, and many others. However, whenever there is a Δ4 double bond, it adopts the trans (or E) configuration. We synthesized a ceramide containing 4 Z-sphingosine and palmitic acid ( cis-pCer) and studied its behavior in the form of monolayers extended on an air-water interface. cis-pCer acted very differently from the trans isomer in that, upon lateral compression of the monolayer, a solid-solid transition was clearly observed at a mean molecular area ≤44 Ų·molecule^-1, whose characteristics depended on the rate of compression. The solid-solid transition, as well as states of domain coexistence, could be imaged by atomic force microscopy and by Brewster-angle microscopy. Atomistic molecular dynamics simulations provided results compatible with the experimentally observed differences between the cis and trans isomers. The data can help in the exploration of other solid-solid transitions in lipids, both in vitro and in vivo, that have gone up to now undetected because of their less obvious change in surface properties along the transition, as compared to cis-pCer.

GBA-Associated Parkinson's Disease and Other Synucleinopathies

PURPOSE OF REVIEW

GBA mutations are the most common known genetic cause of Parkinson's disease (PD). Its biological pathway may be important in idiopathic PD, since activity of the enzyme encoded by GBA, glucocerebrosidase, is reduced even among PD patients without GBA mutations. This article describes the structure and function of GBA, reviews recent literature on the clinical phenotype of GBA PD, and suggests future directions for research, counseling, and treatment.

RECENT FINDINGS

Several longitudinal studies have shown that GBA PD has faster motor and cognitive progression than idiopathic PD and that this effect is dose dependent. New evidence suggests that GBA mutations may be important in multiple system atrophy. Further, new interventional studies focusing on GBA PD are described. These studies may increase the interest of PD patients and caregivers in genetic counseling. GBA mutation status may help clinicians estimate PD progression, though mechanisms underlying GBA and synucleinopathy require further understanding.

The Neutral Sphingomyelinase 2 Is Required to Polarize and Sustain T Cell Receptor Signaling

By promoting ceramide release at the cytosolic membrane leaflet, the neutral sphingomyelinase 2 (NSM) is capable of organizing receptor and signalosome segregation. Its role in T cell receptor (TCR) signaling remained so far unknown. We now show that TCR-driven NSM activation is dispensable for TCR clustering and initial phosphorylation, but of crucial importance for further signal amplification. In particular, at low doses of TCR stimulatory antibodies, NSM is required for Ca²⁺ mobilization and T cell proliferation. NSM-deficient T cells lack sustained CD3ζ and ZAP-70 phosphorylation and are unable to polarize and stabilize their microtubular system. We identified PKCζ as the key NSM downstream effector in this second wave of TCR signaling supporting dynamics of microtubule-organizing center (MTOC). Ceramide supplementation rescued PKCζ membrane recruitment and MTOC translocation in NSM-deficient cells. These findings identify the NSM as essential in TCR signaling when dynamic cytoskeletal reorganization promotes continued lateral and vertical supply of TCR signaling components: CD3ζ, Zap70, and PKCζ, and functional immune synapses are organized and stabilized via MTOC polarization.

Incorporation studies of clickable ceramides in Jurkat cell plasma membranes

The incorporation properties of ceramide analogues for click chemistry in Jurkat T cells were investigated. The analogues varied in the acyl chain length and the position of the functional group for click chemistry. Fluorescence microscopy studies including anisotropy and quenching experiments showed significant differences in the accessibility of the functional group indicating different incorporation properties into the plasma membrane.