Sphingomyelin synthases regulate protein trafficking and secretion

Metabolic Signatures Elucidate the Effect of Body Mass Index on Type 2 Diabetes

Obesity plays an important role in the development of insulin resistance and diabetes, but the molecular mechanism that links obesity and diabetes is still not completely understood. Here, we used 146 targeted metabolomic profiles from the German KORA FF4 cohort consisting of 1715 participants and associated them with obesity and type 2 diabetes. In the basic model, 83 and 51 metabolites were significantly associated with body mass index (BMI) and T2D, respectively. Those metabolites are branched-chain amino acids, acylcarnitines, lysophospholipids, or phosphatidylcholines. In the full model, 42 and 3 metabolites were significantly associated with BMI and T2D, respectively, and replicate findings in the previous studies. Sobel mediation testing suggests that the effect of BMI on T2D might be mediated via lipids such as sphingomyelin (SM) C16:1, SM C18:1 and diacylphosphatidylcholine (PC aa) C38:3. Moreover, mendelian randomization suggests a causal relationship that BMI causes the change of SM C16:1 and PC aa C38:3, and the change of SM C16:1, SM C18:1, and PC aa C38:3 contribute to T2D incident. Biological pathway analysis in combination with genetics and mice experiments indicate that downregulation of sphingolipid or upregulation of phosphatidylcholine metabolism is a causal factor in early-stage T2D pathophysiology. Our findings indicate that metabolites like SM C16:1, SM C18:1, and PC aa C38:3 mediate the effect of BMI on T2D and elucidate their role in obesity related T2D pathologies.

Membrane Sphingomyelin in Host Cells Is Essential for Nucleocapsid Penetration into the Cytoplasm after Hemifusion during Rubella Virus Entry

The lipid composition of the host cell membrane is one of the key determinants of the entry of enveloped viruses into cells. To elucidate the detailed mechanisms behind the cell entry of rubella virus (RuV), one of the enveloped viruses, we searched for host factors involved in such entry by using CRISPR/Cas9 genome-wide knockout screening, and we found sphingomyelin synthase 1 (SMS1), encoded by the SGMS1 gene, as a candidate. RuV growth was strictly suppressed in SGMS1-knockout cells and was completely recovered by the overexpression of enzymatically active SMS1 and partially recovered by that of SMS2, another member of the SMS family, but not by that of enzymatically inactive SMS1. An entry assay using pseudotyped vesicular stomatitis virus possessing RuV envelope proteins revealed that sphingomyelin generated by SMSs is crucial for at least RuV entry. In SGMS1-knockout cells, lipid mixing between the RuV envelope membrane and the membrane of host cells occurred, but entry of the RuV genome from the viral particles into the cytoplasm was strongly inhibited. This indicates that sphingomyelin produced by SMSs is essential for the formation of membrane pores after hemifusion occurs during RuV entry. IMPORTANCE Infection with rubella virus during pregnancy causes congenital rubella syndrome in infants. Despite its importance in public health, the detailed mechanisms of rubella virus cell entry have only recently become somewhat clearer. The E1 protein of rubella virus is classified as a class II fusion protein based on its structural similarity, but it has the unique feature that its activity is dependent on calcium ion binding in the fusion loops. In this study, we found another unique feature, as cellular sphingomyelin plays a critical role in the penetration of the nucleocapsid into the cytoplasm after hemifusion by rubella virus. This provides important insight into the entry mechanism of rubella virus. This study also presents a model of hemifusion arrest during cell entry by an intact virus, providing a useful tool for analyzing membrane fusion, a biologically important phenomenon.

Vti1a/b support distinct aspects of TGN and cis-/medial Golgi organization

Retrograde trafficking towards the trans-Golgi network (TGN) is important for dense core vesicle (DCV) biogenesis. Here, we used Vti1a/b deficient neurons to study the impact of disturbed retrograde trafficking on Golgi organization and cargo sorting. In Vti1a/b deficient neurons, staining intensity of cis-/medial Golgi proteins (e.g., GM130 and giantin) was increased, while the intensity of two recycling TGN proteins, TGN38 and TMEM87A, was decreased and the TGN-resident protein Golgin97 was normal. Levels and localization of DCV cargo markers, LAMP1 and KDEL were also altered. This phenotype was not caused by reduced Golgi size or absence of a TGN compartment. The phenotype was partially phenocopied by disturbing sphingolipid homeostasis, but was not rescued by overexpression of sphingomyelin synthases or the sphingolipid synthesis inhibitor myriocin. We conclude that Vti1a/b are important for distinct aspects of TGN and cis-/medial Golgi organization. Our data underline the importance of retrograde trafficking for Golgi organization, DCV cargo sorting and the distribution of proteins of the regulated secretory pathway.

Sphingolipids in plants: a guidebook on their function in membrane architecture, cellular processes, and environmental or developmental responses

Sphingolipids are fundamental lipids involved in various cellular, developmental and stress-response processes. As such, they orchestrate not only vital molecular mechanisms of living cells but also act in diseases, thus qualifying as potential pharmaceutical targets. Sphingolipids are universal to eukaryotes and are also present in some prokaryotes. Some sphingolipid structures are conserved between animals, plants and fungi, whereas others are found only in plants and fungi. In plants, the structural diversity of sphingolipids, as well as their downstream effectors and molecular and cellular mechanisms of action, are of tremendous interest to both basic and applied researchers, as about half of all small molecules in clinical use originate from plants. Here, we review recent advances towards a better understanding of the biosynthesis of sphingolipids, the diversity in their structures as well as their functional roles in membrane architecture, cellular processes such as membrane trafficking and cell polarity, and cell responses to environmental or developmental signals.

Regulation of human sphingomyelin synthase 1 translation through its 5'-untranslated region

Bcr‐abl1 oncogene causes a shift in the transcription start site of the SMS1 gene (SGMS1) encoding the sphingomyelin (SM) synthesizing enzyme, sphingomyelin synthase 1 (SMS1). This results in an mRNA with a significantly shorter 5′‐UTR, called 7‐SGMS1, which is translated more efficiently than another transcript (IIb‐SGMS1) with a longer 5′UTR in Bcr‐abl1‐positive cells. Here, we determine the effects of these alternative 5′UTRs on SMS1 translation and investigate the key features underlying such regulation. First, the presence of the longer IIb 5′UTR is sufficient to greatly impair translation of a reporter gene. Deletion of the upstream open reading frame (−164 nt) or of the predicted stem‐loops in the 5′UTR of IIb‐SGMS1 has minimal effects on SGMS1 translation. Conversely, deletion of nucleotides −310 to −132 enhanced transcription of IIb‐SGMS1 to reach that of 7‐SGMS1. We thus suggest that regulatory features within nucleotides −310 and −132 modulate IIb‐SGMS1 translation efficiency.

Diminished Sphingolipid Metabolism, a Hallmark of Future Type 2 Diabetes Pathogenesis, Is Linked to Pancreatic β Cell Dysfunction

Gestational diabetes mellitus (GDM) is the top risk factor for future type 2 diabetes (T2D) development. Ethnicity profoundly influences who will transition from GDM to T2D, with high risk observed in Hispanic women. To better understand this risk, a nested 1:1 pair-matched, Hispanic-specific, case-control design was applied to a prospective cohort with GDM history. Women who were non-diabetic 6-9 weeks postpartum (baseline) were monitored for the development of T2D. Metabolomics were performed on baseline plasma to identify metabolic pathways associated with T2D risk. Notably, diminished sphingolipid metabolism was highly associated with future T2D. Defects in sphingolipid metabolism were further implicated by integrating metabolomics and genome-wide association data, which identified two significantly enriched T2D-linked genes, CERS2 and CERS4. Follow-up experiments in mice and cells demonstrated that inhibiting sphingolipid metabolism impaired pancreatic β cell function. These data suggest early postpartum alterations in sphingolipid biosynthesis contribute to β cell dysfunction and T2D risk.

Antineoplastic Agents Targeting Sphingolipid Pathways

Emerging studies in the enigmatic area of bioactive lipids have made many exciting new discoveries in recent years. Once thought to play a strictly structural role in cellular function, it has since been determined that sphingolipids and their metabolites perform a vast variety of cellular functions beyond what was previously believed. Of utmost importance is their role in cellular signaling, for it is now well understood that select sphingolipids serve as bioactive molecules that play critical roles in both cancer cell death and survival, as well as other cellular responses such as chronic inflammation, protection from intestinal pathogens, and intrinsic protection from intestinal contents, each of which are associated with oncogenesis. Importantly, it has been demonstrated time and time again that many different tumors display dysregulation of sphingolipid metabolism, and the exact profile of said dysregulation has been proven to be useful in determining not only the presence of a tumor, but also the susceptibility to various chemotherapeutic drugs, as well as the metastasizing characteristics of the malignancies. Since these discoveries surfaced it has become apparent that the understanding of sphingolipid metabolism and profile will likely become of great importance in the clinic for both chemotherapy and diagnostics of cancer. The goal of this paper is to provide a comprehensive review of the current state of chemotherapeutic agents that target sphingolipid metabolism that are undergoing clinical trials. Additionally, we will formulate questions involving the use of sphingolipid metabolism as chemotherapeutic targets in need of further research.

Concerted regulation of actin polymerization during constitutive secretion by cortactin and PKD2

Constitutive secretion from the trans-Golgi-network (TGN) is facilitated by a concerted regulation of vesicle biogenesis and fission processes. The protein kinase D family (PKD) has been previously described to enhance vesicle fission by modifying the lipid environment. PKD also phosphorylates the actin regulatory protein cortactin at S298 to impair synergistic actin polymerization. We here report additional functions for PKD2 (also known as PRKD2) and cortactin in the regulation of actin polymerization during the fission of transport carriers from the TGN. Phosphorylation of cortactin at S298 impairs the interaction between WIP (also known as WIPF1) and cortactin. WIP stabilizes the autoinhibited conformation of N-WASP (also known as WASL). This leads to an inhibition of synergistic Arp2/3-complex-dependent actin polymerization at the TGN. PKD2 activity at the TGN is controlled by active CDC42-GTP which directly activates N-WASP, inhibits PKD2 and shifts the balance to non-S298-phosphorylated cortactin, which can in turn sequester WIP from N-WASP. Consequently, synergistic actin polymerization at the TGN and constitutive secretion are enhanced.

Lipid-dependent coupling of secretory cargo sorting and trafficking at the trans-Golgi network

In eukaryotic cells, the trans-Golgi network (TGN) serves as a platform for secretory cargo sorting and trafficking. In recent years, it has become evident that a complex network of lipid–lipid and lipid–protein interactions contributes to these key functions. This review addresses the role of lipids at the TGN with a particular emphasis on sphingolipids and diacylglycerol. We further highlight how these lipids couple secretory cargo sorting and trafficking for spatiotemporal coordination of protein transport to the plasma membrane.

The discovery of novel predictive biomarkers and early-stage pathophysiology for the transition from gestational diabetes to type 2 diabetes

AIMS/HYPOTHESIS

Gestational diabetes mellitus (GDM) affects up to 20% of pregnancies, and almost half of the women affected progress to type 2 diabetes later in life, making GDM the most significant risk factor for the development of future type 2 diabetes. An accurate prediction of future type 2 diabetes risk in the early postpartum period after GDM would allow for timely interventions to prevent or delay type 2 diabetes. In addition, new targets for interventions may be revealed by understanding the underlying pathophysiology of the transition from GDM to type 2 diabetes. The aim of this study is to identify both a predictive signature and early-stage pathophysiology of the transition from GDM to type 2 diabetes.

METHODS

We used a well-characterised prospective cohort of women with a history of GDM pregnancy, all of whom were enrolled at 6-9 weeks postpartum (baseline), were confirmed not to have diabetes via 2 h 75 g OGTT and tested anually for type 2 diabetes on an ongoing basis (2 years of follow-up). A large-scale targeted lipidomic study was implemented to analyse ~1100 lipid metabolites in baseline plasma samples using a nested pair-matched case-control design, with 55 incident cases matched to 85 non-case control participants. The relationships between the concentrations of baseline plasma lipids and respective follow-up status (either type 2 diabetes or no type 2 diabetes) were employed to discover both a predictive signature and the underlying pathophysiology of the transition from GDM to type 2 diabetes. In addition, the underlying pathophysiology was examined in vivo and in vitro.

RESULTS

Machine learning optimisation in a decision tree format revealed a seven-lipid metabolite type 2 diabetes predictive signature with a discriminating power (AUC) of 0.92 (87% sensitivity, 93% specificity and 91% accuracy). The signature was highly robust as it includes 45-fold cross-validation under a high confidence threshold (1.0) and binary output, which together minimise the chance of data overfitting and bias selection. Concurrent analysis of differentially expressed lipid metabolite pathways uncovered the upregulation of α-linolenic/linoleic acid metabolism (false discovery rate [FDR] 0.002) and fatty acid biosynthesis (FDR 0.005) and the downregulation of sphingolipid metabolism (FDR 0.009) as being strongly associated with the risk of developing future type 2 diabetes. Focusing specifically on sphingolipids, the downregulation of sphingolipid metabolism using the pharmacological inhibitors fumonisin B1 (FB1) and myriocin in mouse islets and Min6 K8 cells (a pancreatic beta-cell like cell line) significantly impaired glucose-stimulated insulin secretion but had no significant impact on whole-body glucose homeostasis or insulin sensitivity.

CONCLUSIONS/INTERPRETATION

We reveal a novel predictive signature and associate reduced sphingolipids with the pathophysiology of transition from GDM to type 2 diabetes. Attenuating sphingolipid metabolism in islets impairs glucose-stimulated insulin secretion.

Exploring new routes for secretory protein export from the trans-Golgi network

Sorting of soluble proteins for transport to intracellular compartments and for secretion from cells is essential for cell and tissue homeostasis. The trans-Golgi network (TGN) is a major sorting station that sorts secretory proteins into specific carriers to transport them to their final destinations. The sorting of lysosomal hydrolases at the TGN by the mannose 6-phosphate receptor is well understood. The recent discovery of a Ca²⁺-based sorting of secretory cargo at the TGN is beginning to uncover the mechanism by which cells sort secretory cargoes from Golgi residents and cargoes destined to the other cellular compartments. This Ca²⁺-based sorting involves the cytoplasmic actin cytoskeleton, which through membrane anchored Ca²⁺ ATPase SPCA1 and the luminal Ca²⁺ binding protein Cab45 sorts of a subset of secretory proteins at the TGN. We present this discovery and highlight important challenges that remain unaddressed in the overall pathway of cargo sorting at the TGN.

Sphingomyelin synthase 2 promotes an aggressive breast cancer phenotype by disrupting the homoeostasis of ceramide and sphingomyelin

Breast cancer is the most common type of carcinoma in women worldwide, but the mechanisms underlying tumour development and progression remain unclear. Sphingomyelin synthase 2 (SGMS2) is a crucial regulator involved in ceramide (Cer) and sphingomyelin (SM) homoeostasis that is mostly studied for its role in lipid metabolism. Our primary study indicated that high SGMS2 expression is associated with breast cancer metastasis. Gain- and loss-of-function assays in vitro and in vivo revealed that SGMS2 promotes cancer cell proliferation by suppressing apoptosis through a Cer-associated pathway and promotes cancer cell invasiveness by enhancing epithelial-to-mesenchymal transition (EMT) initiation through the TGF-β/Smad signalling pathway. Further study determined that SGMS2 activated the TGF-β/Smad signalling pathway primarily by increasing TGF-β1 secretion, which was likely associated with aberrant expression of SM. Thus, our findings indicate that SGMS2-mediated activation of the TGF-β/Smad signalling pathway is important in breast cancer progression, which provides new insight into the mechanisms underlying breast cancer metastasis and suggests a possible anticancer therapy for breast cancer.

Activity of the SPCA1 Calcium Pump Couples Sphingomyelin Synthesis to Sorting of Secretory Proteins in the Trans-Golgi Network

How the principal functions of the Golgi apparatus-protein processing, lipid synthesis, and sorting of macromolecules-are integrated to constitute cargo-specific trafficking pathways originating from the trans-Golgi network (TGN) is unknown. Here, we show that the activity of the Golgi localized SPCA1 calcium pump couples sorting and export of secreted proteins to synthesis of new lipid in the TGN membrane. A secreted Ca2+-binding protein, Cab45, constitutes the core component of a Ca2+-dependent, oligomerization-driven sorting mechanism whereby secreted proteins bound to Cab45 are packaged into a TGN-derived vesicular carrier whose membrane is enriched in sphingomyelin, a lipid implicated in TGN-to-cell surface transport. SPCA1 activity is controlled by the sphingomyelin content of the TGN membrane, such that local sphingomyelin synthesis promotes Ca2+ flux into the lumen of the TGN, which drives secretory protein sorting and export, thereby establishing a protein- and lipid-specific secretion pathway.

Defective cortex glia plasma membrane structure underlies light-induced epilepsy in cpes mutants

Seizures induced by visual stimulation (photosensitive epilepsy; PSE) represent a common type of epilepsy in humans, but the molecular mechanisms and genetic drivers underlying PSE remain unknown, and no good genetic animal models have been identified as yet. Here, we show an animal model of PSE, in Drosophila, owing to defective cortex glia. The cortex glial membranes are severely compromised in ceramide phosphoethanolamine synthase (cpes)-null mutants and fail to encapsulate the neuronal cell bodies in the Drosophila neuronal cortex. Expression of human sphingomyelin synthase 1, which synthesizes the closely related ceramide phosphocholine (sphingomyelin), rescues the cortex glial abnormalities and PSE, underscoring the evolutionarily conserved role of these lipids in glial membranes. Further, we show the compromise in plasma membrane structure that underlies the glial cell membrane collapse in cpes mutants and leads to the PSE phenotype.

Analysis of the Myc-induced pancreatic β cell islet tumor microenvironment using imaging ToF-SIMS

Solid tumors are a structurally complex system, composed of many different cell types. The tumor microenvironment includes nonmalignant cell types that participate in complex interactions with tumor cells. The cross talk between tumor and normal cells is implicated in regulating cell growth, metastatic potential, and chemotherapeutic drug resistance. A new approach is required to interrogate and quantitatively characterize cell to cell interactions in this complex environment. Here, the authors have applied time-of-flight secondary ion mass spectrometry (ToF-SIMS) to analyze Myc-induced pancreatic β cell islet tumors. The high mass resolution and micron spatial resolution of ToF-SIMS allows detection of metabolic intermediates such as lipids and amino acids. Employing multivariate analysis, specifically, principal component analysis, the authors show that it is possible to chemically distinguish cancerous islets from normal tissue, in addition to intratumor heterogeneity. These heterogeneities can then be imaged and investigated using another modality such as sum harmonic generation microscopy. Using these techniques with a specialized mouse model, the authors found significant metabolic changes occurring within β cell tumors and the surrounding tissues. Specific alterations of the lipid, amino acid, and nucleotide metabolism were observed, demonstrating that ToF-SIMS can be utilized to identify large-scale changes that occur in the tumor microenvironment and could thereby increase the understanding of tumor progression and the tumor microenvironment.

Role and Function of Sphingomyelin Biosynthesis in the Development of Cancer

Sphingomyelin (SM) biosynthesis represents a complex, finely regulated process, mostly occurring in vertebrates. It is intimately linked to lipid transport and it is ultimately carried out by two enzymes, SM synthase 1 and 2, selectively localized in the Golgi and plasma membrane. In the course of the SM biosynthetic reaction, various lipids are metabolized. Because these lipids have both structural and signaling functions, the SM biosynthetic process has the potential to affect diverse important cellular processes (such as cell proliferation, cell survival, and migration). Thus defects in SM biosynthesis might directly or indirectly impact the normal physiology of the cell and eventually of the organism. In this chapter, we will focus on evidence supporting a role for SM biosynthesis in specific cellular functions and how its dysregulation can affect neoplastic transformation.

Sphingolipid abnormalities in cancer multidrug resistance: Chicken or egg?

The cancer multidrug resistance (MDR) phenotype encompasses a myriad of molecular, genetic and cellular alterations resulting from progressive oncogenic transformation and selection. Drug efflux transporters, in particular the MDR P-glycoprotein ABCB1, play an important role in MDR but cannot confer the complete phenotype alone indicating parallel alterations are prerequisite. Sphingolipids are essential constituents of lipid raft domains and directly participate in functionalization of transmembrane proteins, including providing an optimal lipid microenvironment for multidrug transporters, and are also perturbed in cancer. Here we postulate that increased sphingomyelin content, developing early in some cancers, recruits and functionalizes plasma membrane ABCB1 conferring a state of partial MDR, which is completed by glycosphingolipid disturbance and the appearance of intracellular vesicular ABCB1. In this review, the independent and interdependent roles of sphingolipid alterations and ABCB1 upregulation during the transformation process and resultant conferment of partial and complete MDR phenotypes are discussed.