Toxoplasma ceramide synthases: Gene duplication, functional divergence, and roles in parasite fitness.

Toxoplasma gondii is an obligate, intracellular apicomplexan protozoan parasite of both humans and animals that can cause fetal damage and abortion and severe disease in the immunosuppressed. Sphingolipids have indispensable functions as signaling molecules and are essential and ubiquitous components of eukaryotic membranes that are both synthesized and scavenged by the Apicomplexa. Ceramide is the precursor for all sphingolipids, and here we report the identification, localization and analyses of the Toxoplasma ceramide synthases TgCerS1 and TgCerS2. Interestingly, we observed that while TgCerS1 was a fully functional orthologue of the yeast ceramide synthase (Lag1p) capable of catalyzing the conversion of sphinganine to ceramide, in contrast TgCerS2 was catalytically inactive. Furthermore, genomic deletion of TgCerS1 using CRISPR/Cas-9 led to viable but slow-growing parasites indicating its importance but not indispensability. In contrast, genomic knock out of TgCerS2 was only accessible utilizing the rapamycin-inducible Cre recombinase system. Surprisingly, the results demonstrated that this "pseudo" ceramide synthase, TgCerS2, has a considerably greater role in parasite fitness than its catalytically active orthologue (TgCerS1). Phylogenetic analyses indicated that, as in humans and plants, the ceramide synthase isoforms found in Toxoplasma and other Apicomplexa may have arisen through gene duplication. However, in the Apicomplexa the duplicated copy is hypothesized to have subsequently evolved into a non-functional "pseudo" ceramide synthase. This arrangement is unique to the Apicomplexa and further illustrates the unusual biology that characterize these protozoan parasites.

Membrane damage and repair: a thin line between life and death.

Bilayered membranes separate cells from their surroundings and form boundaries between intracellular organelles and the cytosol. Gated transport of solutes across membranes enables cells to establish vital ion gradients and a sophisticated metabolic network. However, an advanced compartmentalization of biochemical reactions makes cells also particularly vulnerable to membrane damage inflicted by pathogens, chemicals, inflammatory responses or mechanical stress. To avoid potentially lethal consequences of membrane injuries, cells continuously monitor the structural integrity of their membranes and readily activate appropriate pathways to plug, patch, engulf or shed the damaged membrane area. Here, we review recent insights into the cellular mechanisms that underly an effective maintenance of membrane integrity. We discuss how cells respond to membrane lesions caused by bacterial toxins and endogenous pore-forming proteins, with a primary focus on the intimate crosstalk between membrane proteins and lipids during wound formation, detection and elimination. We also discuss how a delicate balance between membrane damage and repair determines cell fate upon bacterial infection or activation of pro-inflammatory cell death pathways.

Biofunctional Nanodot Arrays in Living Cells Uncover Synergistic Co-Condensation of Wnt Signalodroplets.

Qualitative and quantitative analysis of transient signaling platforms in the plasma membrane has remained a key experimental challenge. Here, biofunctional nanodot arrays (bNDAs) are developed to spatially control dimerization and clustering of cell surface receptors at the nanoscale. High-contrast bNDAs with spot diameters of ≈300 nm are obtained by capillary nanostamping of bovine serum albumin bioconjugates, which are subsequently biofunctionalized by reaction with tandem anti-green fluorescence protein (GFP) clamp fusions. Spatially controlled assembly of active Wnt signalosomes is achieved at the nanoscale in the plasma membrane of live cells by capturing the co-receptor Lrp6 into bNDAs via an extracellular GFP tag. Strikingly, co-recruitment is observed of co-receptor Frizzled-8 as well as the cytosolic scaffold proteins Axin-1 and Disheveled-2 into Lrp6 nanodots in the absence of ligand. Density variation and the high dynamics of effector proteins uncover highly cooperative liquid-liquid phase separation (LLPS)-driven assembly of Wnt "signalodroplets" at the plasma membrane, pinpointing the synergistic effects of LLPS for Wnt signaling amplification. These insights highlight the potential of bNDAs for systematically interrogating nanoscale signaling platforms and condensation at the plasma membrane of live cells.

Lipid landscapes and pipelines in membrane homeostasis.

The lipid composition of cellular organelles is tailored to suit their specialized tasks. A fundamental transition in the lipid landscape divides the secretory pathway in early and late membrane territories, allowing an adaptation from biogenic to barrier functions. Defending the contrasting features of these territories against erosion by vesicular traffic poses a major logistical problem. To this end, cells evolved a network of lipid composition sensors and pipelines along which lipids are moved by non-vesicular mechanisms. We review recent insights into the molecular basis of this regulatory network and consider examples in which malfunction of its components leads to system failure and disease.

Diverting CERT-mediated ceramide transport to mitochondria triggers Bax-dependent apoptosis.

A deregulation of ceramide biosynthesis in the endoplasmic reticulum (ER) is frequently linked to induction of mitochondrial apoptosis. Although in vitro studies suggest that ceramides might initiate cell death by acting directly on mitochondria, their actual contribution to the apoptotic response in living cells is unclear. Here, we have analyzed the consequences of targeting the biosynthetic flow of ceramides to mitochondria using a ceramide transfer protein (encoded by COL4A3BP) equipped with an OMM anchor, mitoCERT. Cells expressing mitoCERT import ceramides into mitochondria and undergo Bax-dependent apoptosis. Apoptosis induction by mitoCERT was abolished through (i) removal of its ceramide transfer domain, (ii) disruption of its interaction with VAMP-associated proteins (VAPs) in the ER, (iii) addition of antagonistic CERT inhibitor HPA12, (iv) blocking de novo ceramide synthesis and (v) targeting of a bacterial ceramidase to mitochondria. Our data provide the first demonstration that translocation of ER ceramides to mitochondria specifically commits cells to death and establish mitoCERT as a valuable new tool to unravel the molecular principles underlying ceramide-mediated apoptosis.

Membrane contact sites, ancient and central hubs of cellular lipid logistics.

Membrane contact sites (MCSs) are regions where two organelles are closely apposed to facilitate molecular communication and promote a functional integration of compartmentalized cellular processes. There is growing evidence that MCSs play key roles in controlling intracellular lipid flows and distributions. Strikingly, even organelles connected by vesicular trafficking exchange lipids en bulk via lipid transfer proteins that operate at MCSs. Herein, we describe how MCSs developed into central hubs of lipid logistics during the evolution of eukaryotic cells. We then focus on how modern eukaryotes exploit MCSs to help solve a major logistical problem, namely to preserve the unique lipid mixtures of their early and late secretory organelles in the face of extensive vesicular trafficking. This article is part of a Special Issue entitled: Membrane Contact Sites edited by Christian Ungermann and Benoit Kornmann.

A search for ceramide binding proteins using bifunctional lipid analogs yields CERT-related protein StarD7.

Ceramides are central intermediates of sphingolipid metabolism with dual roles as mediators of cellular stress signaling and mitochondrial apoptosis. How ceramides exert their cytotoxic effects is unclear and their poor solubility in water hampers a search for specific protein interaction partners. Here, we report the application of a photoactivatable and clickable ceramide analog, pacCer, to identify ceramide binding proteins and unravel the structural basis by which these proteins recognize ceramide. Besides capturing ceramide transfer protein (CERT) from a complex proteome, our approach yielded CERT-related steroidogenic acute regulatory protein D7 (StarD7) as novel ceramide binding protein. Previous work revealed that StarD7 is required for efficient mitochondrial import of phosphatidylcholine (PC) and serves a critical role in mitochondrial function and morphology. Combining site-directed mutagenesis and photoaffinity labeling experiments, we demonstrate that the steroidogenic acute regulatory transfer domain of StarD7 harbors a common binding site for PC and ceramide. While StarD7 lacks robust ceramide transfer activity in vitro, we find that its ability to shuttle PC between model membranes is specifically affected by ceramides. Besides demonstrating the suitability of pacCer as a tool to hunt for ceramide binding proteins, our data point at StarD7 as a candidate effector protein by which ceramides may exert part of their mitochondria-mediated cytotoxic effects.

Switching head group selectivity in mammalian sphingolipid biosynthesis by active-site-engineering of sphingomyelin synthases.

SM is a fundamental component of mammalian cell membranes that contributes to mechanical stability, signaling, and sorting. Its production involves the transfer of phosphocholine from phosphatidylcholine onto ceramide, a reaction catalyzed by SM synthase (SMS)1 in the Golgi and SMS2 at the plasma membrane. Mammalian cells also synthesize trace amounts of the SM analog, ceramide phosphoethanolamine (CPE), but the physiological relevance of CPE production is unclear. Previous work revealed that SMS2 is a bifunctional enzyme producing both SM and CPE, whereas a closely related enzyme, SMS-related protein (SMSr)/SAMD8, acts as a monofunctional CPE synthase in the endoplasmic reticulum. Using domain swapping and site-directed mutagenesis on enzymes expressed in defined lipid environments, we here identified structural determinants that mediate the head group selectivity of SMS family members. Notably, a single residue adjacent to the catalytic histidine in the third exoplasmic loop profoundly influenced enzyme specificity, with Glu permitting SMS-catalyzed CPE production and Asp confining the enzyme to produce SM. An exchange of exoplasmic residues with SMSr proved sufficient to convert SMS1 into a bulk CPE synthase. This allowed us to establish mammalian cells that produce CPE rather than SM as the principal phosphosphingolipid and provide a model of the molecular interactions that impart catalytic specificity among SMS enzymes.

Monitoring Changes in the Oligomeric State of a Candidate Endoplasmic Reticulum (ER) Ceramide Sensor by Single-molecule Photobleaching.

Single-molecule photobleaching has emerged as a powerful non-invasive approach to extract the stoichiometry of multimeric membrane proteins in their native cellular environment. However, this method has mainly been used to determine the subunit composition of ion channels and receptors at the plasma membrane. Here, we applied single-molecule photobleaching to analyze the oligomeric state of an endoplasmic reticulum (ER) resident candidate ceramide sensor protein, SMSr/SAMD8. Co-immunoprecipitation and chemical cross-linking studies previously revealed that the N-terminal sterile alpha motif (or SAM) domain of SMSr drives self-assembly of the protein into oligomers and that SMSr oligomerization is promoted by curcumin, a drug known to perturb ER ceramide and calcium homeostasis. Application of cell spreading surface-active coating materials in combination with total internal reflection fluorescence (TIRF) microscopy allowed us to image GFP-tagged SMSr proteins as single fluorescent spots in the ER of HeLa cells in which expression of endogenous SMSr was abolished. In line with our biochemical analysis, we find that the number of bleaching steps in SMSr-GFP-positive spots displays a substantial drop after removal of the SAM domain. In contrast, treatment of cells with curcumin increased the number of bleaching steps. Our results document the first successful application of single-molecule photobleaching to resolve drug-induced and domain-dependent changes in the oligomeric state of an ER-resident membrane protein, hence establishing a complementary method to unravel the mechanism by which SMSr controls ceramide levels in the ER.

Inner workings and biological impact of phospholipid flippases.

The plasma membrane, trans-Golgi network and endosomal system of eukaryotic cells are populated with flippases that hydrolyze ATP to help establish asymmetric phospholipid distributions across the bilayer. Upholding phospholipid asymmetry is vital to a host of cellular processes, including membrane homeostasis, vesicle biogenesis, cell signaling, morphogenesis and migration. Consequently, defining the identity of flippases and their biological impact has been the subject of intense investigations. Recent work has revealed a remarkable degree of kinship between flippases and cation pumps. In this Commentary, we review emerging insights into how flippases work, how their activity is controlled according to cellular demands, and how disrupting flippase activity causes system failure of membrane function, culminating in membrane trafficking defects, aberrant signaling and disease.

Switching head group selectivity in mammalian sphingolipid biosynthesis by active-site engineering of sphingomyelin synthases.

SM is a fundamental component of mammalian cell membranes that contributes to mechanical stability, signaling, and sorting. Its production involves the transfer of phosphocholine from phosphatidylcholine onto ceramide, a reaction catalyzed by SM synthase (SMS) 1 in the Golgi and SMS2 at the plasma membrane. Mammalian cells also synthesize trace amounts of the SM analog ceramide phosphoethanolamine (CPE), but the physiological relevance of CPE production is unclear. Previous work revealed that SMS2 is a bifunctional enzyme producing both SM and CPE, whereas a closely related enzyme, sphingomyelin synthase-related protein (SMSr)/SAMD8, acts as a monofunctional CPE synthase in the endoplasmatic reticulum. Using domain swapping and site-directed mutagenesis on enzymes expressed in defined lipid environments, we here identified structural determinants that mediate head group selectivity of SMS family members. Notably, a single residue adjacent to the catalytic histidine in the third exoplasmic loop profoundly influenced enzyme specificity, with glutamic acid permitting SMS-catalyzed CPE production and aspartic acid confining the enzyme to produce SM. An exchange of exoplasmic residues with SMSr proved sufficient to convert SMS1 into a bulk CPE synthase. This allowed us to establish mammalian cells that produce CPE rather than SM as the principal phosphosphingolipid and provide a model of the molecular interactions that impart catalytic specificity among SMS enzymes.

Sphingomyelin synthase-related protein SMSr is a suppressor of ceramide-induced mitochondrial apoptosis.

Cells synthesize ceramides in the endoplasmic reticulum (ER) as precursors for sphingolipids to form an impermeable plasma membrane. As ceramides are engaged in apoptotic pathways, cells would need to monitor their levels closely to avoid killing themselves during sphingolipid biosynthesis. How this is accomplished remains to be established. Here we identify SMSr (SAMD8), an ER-resident ceramide phosphoethanolamine (CPE) synthase, as a suppressor of ceramide-mediated cell death. Disruption of SMSr catalytic activity causes a rise in ER ceramides and their mislocalization to mitochondria, triggering a mitochondrial pathway of apoptosis. Blocking de novo ceramide synthesis, stimulating ceramide export from the ER or targeting a bacterial ceramidase to mitochondria rescues SMSr-deficient cells from apoptosis. We also show that SMSr-catalyzed CPE production, although essential, is not sufficient to suppress ceramide-induced cell death and that SMSr-mediated ceramide homeostasis requires the N-terminal sterile α-motif, or SAM domain, of the enzyme. These results define ER ceramides as bona fide transducers of mitochondrial apoptosis and indicate a primary role of SMSr in monitoring ER ceramide levels to prevent inappropriate cell death during sphingolipid biosynthesis.

Functional characterization of enzymes catalyzing ceramide phosphoethanolamine biosynthesis in mice.

Besides bulk amounts of SM, mammalian cells produce small quantities of the SM analog ceramide phosphoethanolamine (CPE). Little is known about the biological role of CPE or enzymes responsible for CPE production. Heterologous expression studies revealed that SM synthase (SMS)2 is a bifunctional enzyme producing both SM and CPE, whereas SMS-related protein (SMSr) serves as monofunctional CPE synthase. Acute disruption of SMSr catalytic activity in cultured cells causes a rise in endoplasmic reticulum (ER) ceramides, fragmentation of ER exit sites, and induction of mitochondrial apoptosis. To address the relevance of CPE biosynthesis in vivo, we analyzed the tissue-specific distribution of CPE in mice and generated mouse lines lacking SMSr and SMS2 catalytic activity. We found that CPE levels were >300-fold lower than SM in all tissues examined. Unexpectedly, combined inactivation of SMSr and SMS2 significantly reduced, but did not eliminate, tissue-specific CPE pools and had no obvious impact on mouse development or fertility. While SMSr is widely expressed and serves as the principal CPE synthase in the brain, blocking its catalytic activity did not affect ceramide levels or secretory pathway integrity in the brain or any other tissue. Our data provide a first inventory of CPE species and CPE-biosynthetic enzymes in mammals.

Fat & fabulous: bifunctional lipids in the spotlight.

Understanding biological processes at the mechanistic level requires a systematic charting of the physical and functional links between all cellular components. While protein-protein and protein-nucleic acid networks have been subject to many global surveys, other critical cellular components such as membrane lipids have rarely been studied in large-scale interaction screens. Here, we review the development of photoactivatable and clickable lipid analogues-so-called bifunctional lipids-as novel chemical tools that enable a global profiling of lipid-protein interactions in biological membranes. Recent studies indicate that bifunctional lipids hold great promise in systematic efforts to dissect the elaborate crosstalk between proteins and lipids in live cells and organisms. This article is part of a Special Issue entitled Tools to study lipid functions.

Ceramide phosphoethanolamine biosynthesis in Drosophila is mediated by a unique ethanolamine phosphotransferase in the Golgi lumen.

Sphingomyelin (SM) is a vital component of mammalian membranes, providing mechanical stability and a structural framework for plasma membrane organization. Its production involves the transfer of phosphocholine from phosphatidylcholine onto ceramide, a reaction catalyzed by SM synthase in the Golgi lumen. Drosophila lacks SM and instead synthesizes the SM analogue ceramide phosphoethanolamine (CPE) as the principal membrane sphingolipid. The corresponding CPE synthase shares mechanistic features with enzymes mediating phospholipid biosynthesis via the Kennedy pathway. Using a functional cloning strategy, we here identified a CDP-ethanolamine:ceramide ethanolamine phosphotransferase as the enzyme responsible for CPE production in Drosophila. CPE synthase constitutes a new branch within the CDP-alcohol phosphotransferase superfamily with homologues in Arthropoda (insects, spiders, mites, scorpions), Cnidaria (Hydra, sea anemones), and Mollusca (oysters) but not in most other animal phyla. The enzyme resides in the Golgi complex with its active site facing the lumen, contrary to the membrane topology of other CDP-alcohol phosphotransferases. Our findings open up an important new avenue to address the biological role of CPE, an enigmatic membrane constituent of a wide variety of invertebrate and marine organisms.

Mapping functional interactions in a heterodimeric phospholipid pump.

Type 4 P-type ATPases (P(4)-ATPases) catalyze phospholipid transport to generate phospholipid asymmetry across membranes of late secretory and endocytic compartments, but their kinship to cation-transporting P-type transporters raised doubts about whether P(4)-ATPases alone are sufficient to mediate flippase activity. P(4)-ATPases form heteromeric complexes with Cdc50 proteins. Studies of the enzymatic properties of purified P(4)-ATPase·Cdc50 complexes showed that catalytic activity depends on direct and specific interactions between Cdc50 subunit and transporter, whereas in vivo interaction assays suggested that the binding affinity for each other fluctuates during the transport reaction cycle. The structural determinants that govern this dynamic association remain to be established. Using domain swapping, site-directed, and random mutagenesis approaches, we here show that residues throughout the subunit contribute to forming the heterodimer. Moreover, we find that a precise conformation of the large ectodomain of Cdc50 proteins is crucial for the specificity and functionality to transporter/subunit interactions. We also identified two highly conserved disulfide bridges in the Cdc50 ectodomain. Functional analysis of cysteine mutants that disrupt these disulfide bridges revealed an inverse relationship between subunit binding and P(4)-ATPase-catalyzed phospholipid transport. Collectively, our data indicate that a dynamic association between subunit and transporter is crucial for the transport reaction cycle of the heterodimer.

Cellular microcompartments constitute general suborganellar functional units in cells.

All cells are compartmentalized to facilitate enzymatic reactions or cellular dynamics. In eukaryotic cells, organelles differ in their protein/lipid repertoire, luminal ion composition, pH, and redox status. In addition, organelles contain specialized subcompartments even within the same membrane or within its lumen. Moreover, the bacterial plasma membrane reveals a remarkable degree of organization, which is recapitulated in eukaryotic cells and often linked to cell signaling. Finally, protein-based compartments are also known in the bacterial and eukaryotic cytosol. As the organizing principle of such cellular subcompartments is likely similar, previous definitions like rafts, microdomains, and all kinds of '-somes' fall short as a general denominator to describe such suborganellar structures. Within this review, we will introduce the term cellular microcompartment as a general suborganellar functional unit and discuss its relevance to understand subcellular organization and function.

Phospholipids are imported into mitochondria by VDAC, a dimeric beta barrel scramblase.

Mitochondria are double-membrane-bounded organelles that depend critically on phospholipids supplied by the endoplasmic reticulum. These lipids must cross the outer membrane to support mitochondrial function, but how they do this is unclear. We identify the Voltage Dependent Anion Channel (VDAC), an abundant outer membrane protein, as a scramblase-type lipid transporter that catalyzes lipid entry. On reconstitution into membrane vesicles, dimers of human VDAC1 and VDAC2 catalyze rapid transbilayer translocation of phospholipids by a mechanism that is unrelated to their channel activity. Coarse-grained molecular dynamics simulations of VDAC1 reveal that lipid scrambling occurs at a specific dimer interface where polar residues induce large water defects and bilayer thinning. The rate of phospholipid import into yeast mitochondria is an order of magnitude lower in the absence of VDAC homologs, indicating that VDACs provide the main pathway for lipid entry. Thus, VDAC isoforms, members of a superfamily of beta barrel proteins, moonlight as a class of phospholipid scramblases - distinct from alpha-helical scramblase proteins - that act to import lipids into mitochondria.

Systematic analysis of the sphingomyelin synthase family in C. elegans.

Sphingomyelin (SM) is a major component of mammalian cell membranes and particularly abundant in the myelin sheath that surrounds nerve fibers. Its production is catalyzed by SM synthases SMS1 and SMS2, which interconvert phosphatidylcholine and ceramide to diacylglycerol and SM in the Golgi and at the plasma membrane, respectively. As the lipids participating in this reaction fulfill both structural and signaling functions, SMS enzymes have considerable potential to influence diverse important cellular processes. The nematode Caenorhabditis elegans is an attractive model for studying both animal development and human disease. The organism contains five SMS homologues but none of these have been characterized in any detail. Here, we carried out the first systematic analysis of SMS family members in C. elegans . Using heterologous expression systems, genetic ablation, metabolic labeling and lipidome analyses, we show that C. elegans harbors at least three distinct SM synthases and one ceramide phosphoethanolamine (CPE) synthase. Moreover, C. elegans SMS family members have partially overlapping but also unique subcellular distributions and together occupy all principal compartments of the secretory pathway. Our findings shed light on crucial aspects of sphingolipid metabolism in a valuable animal model and opens avenues for exploring the role of SM and its metabolic intermediates in organismal development.

An alliance between lipid transfer proteins and scramblases for membrane expansion.

Membrane growth requires lipid supply, which is usually accomplished by lipid synthesis or vesicular trafficking. In the case of autophagosomes, these principles do not apply. Ghanbarpour et al. postulate that autophagosome expansion relies on non-vesicular lipid delivery from the ER, whereby the activity of a lipid transfer protein (LTP) is directly coupled to scramblase activities in the donor and acceptor bilayers1. This new concept opens the possibility that lipid traffic is controlled by scramblases that provide not only specific docking sites for LTPs, thereby directing lipid flow, but also support their activity by overcoming barriers for lipid extraction and deposition.