Deoxysphingolipids, novel biomarkers for type 2 diabetes, are cytotoxic for insulin-producing cells

Enzymes of sphingolipid metabolism as transducers of metabolic inputs

Sphingolipids (SLs) constitute a discrete subdomain of metabolism, and they display both structural and signaling functions. Accumulating evidence also points to intimate connections between intermediary metabolism and SL metabolism. Given that many SLs exhibit bioactive properties (i.e. transduce signals), these raise the possibility that an important function of SLs is to relay information on metabolic changes into specific cell responses. This could occur at various levels. Some metabolites are incorporated into SLs, whereas others may initiate regulatory or signaling events that, in turn, modulate SL metabolism. In this review, we elaborate on the former as it represents a poorly appreciated aspect of SL metabolism, and we develop the hypothesis that the SL network is highly sensitive to several specific metabolic changes, focusing on amino acids (serine and alanine), various fatty acids, choline (and ethanolamine), and glucose.

Serine and glycine physiology reversibly modulate retinal and peripheral nerve function

Metabolic homeostasis is maintained by redundant pathways to ensure adequate nutrient supply during fasting and other stresses. These pathways are regulated locally in tissues and systemically via the liver, kidney, and circulation. Here, we characterize how serine, glycine, and one-carbon (SGOC) metabolism fluxes across the eye, liver, and kidney sustain retinal amino acid levels and function. Individuals with macular telangiectasia (MacTel), an age-related retinal disease with reduced circulating serine and glycine, carrying deleterious alleles in SGOC metabolic enzymes exhibit an exaggerated reduction in circulating serine. A Phgdh+/- mouse model of this haploinsufficiency experiences accelerated retinal defects upon dietary serine/glycine restriction, highlighting how otherwise silent haploinsufficiencies can impact retinal health. We demonstrate that serine-associated retinopathy and peripheral neuropathy are reversible, as both are restored in mice upon serine supplementation. These data provide molecular insights into the genetic and metabolic drivers of neuro-retinal dysfunction while highlighting therapeutic opportunities to ameliorate this pathogenesis.

(1-Deoxy)ceramides in bilayers containing sphingomyelin and cholesterol

The discovery of a novel sphingolipid subclass, the (1-deoxy)sphingolipids, which lack the 1-hydroxy group, attracted considerable attention in the last decade, mainly due to their involvement in disease. They differed in their physico-chemical properties from the canonical (or 1-hydroxy) sphingolipids and they were more toxic when accumulated in cells, inducing neurodegeneration and other dysfunctions. (1-Deoxy)ceramides, (1-deoxy)dihydroceramides, and (1- deoxymethyl)dihydroceramides, the latter two containing a saturated sphingoid chain, have been studied in this work using differential scanning calorimetry, confocal fluorescence and atomic force microscopy, to evaluate their behavior in bilayers composed of mixtures of three or four lipids. When compared to canonical ceramides (Cer), a C16:0 (1-deoxy)Cer shows a lower miscibility in mixtures of the kind C16:0 sphingomyelin/cholesterol/XCer, where XCer is any (1-deoxy)ceramide, giving rise to the coexistence of a liquid-ordered phase and a gel phase. The latter resembles, in terms of thermotropic behavior and nanomechanical resistance, the gel phase of the C16:0 sphingomyelin/cholesterol/C16:0 Cer mixture [Busto et al., Biophys. J. 2014, 106, 621-630]. Differences are seen between the various C16:0 XCer under study in terms of nanomechanical resistance, bilayer thickness and bilayer topography. When examined in a more fluid environment (bilayers based on C24:1 SM), segregated gel phases are still present. Probably related to such lateral separation, XCer preserve the capacity for membrane permeation, but their effects are significantly lower than those of canonical ceramides. Moreover, C24:1 XCer show significantly lower membrane permeation capacity than their C16:0 counterparts. The above data may be relevant in the pathogenesis of certain sphingolipid-related diseases, including certain neuropathies, diabetes, and glycogen storage diseases.

Characterization of cell cycle, inflammation, and oxidative stress signaling role in non-communicable diseases: Insights into genetic variants, microRNAs and pathways

Non-Communicable Diseases (NCDs) significantly impact global health, contributing to over 70% of premature deaths, as reported by the World Health Organization (WHO). These diseases have complex and multifactorial origins, involving genetic, epigenetic, environmental and lifestyle factors. While Genome-Wide Association Study (GWAS) is widely recognized as a valuable tool for identifying variants associated with complex phenotypes; the multifactorial nature of NCDs necessitates a more comprehensive exploration, encompassing not only the genetic but also the epigenetic aspect. For this purpose, we employed a bioinformatics-multiomics approach to examine the genetic and epigenetic characteristics of NCDs (i.e. colorectal cancer, coronary atherosclerosis, squamous cell lung cancer, psoriasis, type 2 diabetes, and multiple sclerosis), aiming to identify novel biomarkers for diagnosis and prognosis. Leveraging GWAS summary statistics, we pinpointed Single Nucleotide Polymorphisms (SNPs) independently associated with each NCD. Subsequently, we identified genes linked to cell cycle, inflammation and oxidative stress mechanisms, revealing shared genes across multiple diseases, suggesting common functional pathways. From an epigenetic perspective, we identified microRNAs (miRNAs) with regulatory functions targeting these genes of interest. Our findings underscore critical genetic pathways implicated in these diseases. In colorectal cancer, the dysregulation of the "Cytokine Signaling in Immune System" pathway, involving LAMA5 and SMAD7, regulated by Hsa-miR-21-5p, Hsa-miR-103a-3p, and Hsa-miR-195-5p, emerged as pivotal. In coronary atherosclerosis, the pathway associated with "binding of TCF/LEF:CTNNB1 to target gene promoters" displayed noteworthy implications, with the MYC factor controlled by Hsa-miR-16-5p as a potential regulatory factor. Squamous cell lung carcinoma analysis revealed significant pathways such as "PTK6 promotes HIF1A stabilization," regulated by Hsa-let-7b-5p. In psoriasis, the "Endosomal/Vacuolar pathway," involving HLA-C and Hsa-miR-148a-3p and Hsa-miR-148b-3p, was identified as crucial. Type 2 Diabetes implicated the "Regulation of TP53 Expression" pathway, controlled by Hsa-miR-106a-5p and Hsa-miR-106b-5p. In conclusion, our study elucidates the genetic framework and molecular mechanisms underlying NCDs, offering crucial insights into potential genetic/epigenetic biomarkers for diagnosis and prognosis. The specificity of pathways and related miRNAs in different pathologies highlights promising candidates for further clinical validation, with the potential to advance personalized treatments and alleviate the global burden of NCDs.

Circadian organization of lipid landscape is perturbed in type 2 diabetic patients

Lipid homeostasis in humans follows a diurnal pattern in muscle and pancreatic islets, altered upon metabolic dysregulation. We employ tandem and liquid-chromatography mass spectrometry to investigate daily regulation of lipid metabolism in subcutaneous white adipose tissue (SAT) and serum of type 2 diabetic (T2D) and non-diabetic (ND) human volunteers (n = 12). Around 8% of ≈440 lipid metabolites exhibit diurnal rhythmicity in serum and SAT from ND and T2D subjects. The spectrum of rhythmic lipids differs between ND and T2D individuals, with the most substantial changes observed early morning, as confirmed by lipidomics in an independent cohort of ND and T2D subjects (n = 32) conducted at a single morning time point. Strikingly, metabolites identified as daily rhythmic in both serum and SAT from T2D subjects exhibit phase differences. Our study reveals massive temporal and tissue-specific alterations of human lipid homeostasis in T2D, providing essential clues for the development of lipid biomarkers in a temporal manner.

Review of Eukaryote Cellular Membrane Lipid Composition, with Special Attention to the Fatty Acids

Biological membranes, primarily composed of lipids, envelop each living cell. The intricate composition and organization of membrane lipids, including the variety of fatty acids they encompass, serve a dynamic role in sustaining cellular structural integrity and functionality. Typically, modifications in lipid composition coincide with consequential alterations in universally significant signaling pathways. Exploring the various fatty acids, which serve as the foundational building blocks of membrane lipids, provides crucial insights into the underlying mechanisms governing a myriad of cellular processes, such as membrane fluidity, protein trafficking, signal transduction, intercellular communication, and the etiology of certain metabolic disorders. Furthermore, comprehending how alterations in the lipid composition, especially concerning the fatty acid profile, either contribute to or prevent the onset of pathological conditions stands as a compelling area of research. Hence, this review aims to meticulously introduce the intricacies of membrane lipids and their constituent fatty acids in a healthy organism, thereby illuminating their remarkable diversity and profound influence on cellular function. Furthermore, this review aspires to highlight some potential therapeutic targets for various pathological conditions that may be ameliorated through dietary fatty acid supplements. The initial section of this review expounds on the eukaryotic biomembranes and their complex lipids. Subsequent sections provide insights into the synthesis, membrane incorporation, and distribution of fatty acids across various fractions of membrane lipids. The last section highlights the functional significance of membrane-associated fatty acids and their innate capacity to shape the various cellular physiological responses.

Unlocking the Potential: Amino Acids' Role in Predicting and Exploring Therapeutic Avenues for Type 2 Diabetes Mellitus

Diabetes mellitus, particularly type 2 diabetes mellitus (T2DM), imposes a significant global burden with adverse clinical outcomes and escalating healthcare expenditures. Early identification of biomarkers can facilitate better screening, earlier diagnosis, and the prevention of diabetes. However, current clinical predictors often fail to detect abnormalities during the prediabetic state. Emerging studies have identified specific amino acids as potential biomarkers for predicting the onset and progression of diabetes. Understanding the underlying pathophysiological mechanisms can offer valuable insights into disease prevention and therapeutic interventions. This review provides a comprehensive summary of evidence supporting the use of amino acids and metabolites as clinical biomarkers for insulin resistance and diabetes. We discuss promising combinations of amino acids, including branched-chain amino acids, aromatic amino acids, glycine, asparagine and aspartate, in the prediction of T2DM. Furthermore, we delve into the mechanisms involving various signaling pathways and the metabolism underlying the role of amino acids in disease development. Finally, we highlight the potential of targeting predictive amino acids for preventive and therapeutic interventions, aiming to inspire further clinical investigations and mitigate the progression of T2DM, particularly in the prediabetic stage.

Imbalanced unfolded protein response signaling contributes to 1-deoxysphingolipid retinal toxicity

The accumulation of atypical, cytotoxic 1-deoxysphingolipids (1-dSLs) has been linked to retinal diseases such as diabetic retinopathy and Macular Telangiectasia Type 2. However, the molecular mechanisms by which 1-dSLs induce toxicity in retinal cells remain poorly understood. Here, we integrate bulk and single-nucleus RNA-sequencing to define biological pathways that modulate 1-dSL toxicity in human retinal organoids. Our results demonstrate that 1-dSLs differentially activate signaling arms of the unfolded protein response (UPR) in photoreceptor cells and Müller glia. Using a combination of pharmacologic activators and inhibitors, we show that sustained PERK signaling through the integrated stress response (ISR) and deficiencies in signaling through the protective ATF6 arm of the UPR are implicated in 1-dSL-induced photoreceptor toxicity. Further, we demonstrate that pharmacologic activation of ATF6 mitigates 1-dSL toxicity without impacting PERK/ISR signaling. Collectively, our results identify new opportunities to intervene in 1-dSL linked diseases through targeting different arms of the UPR.

Deoxysphingolipids: Atypical Skeletal Muscle Lipids Related to Insulin Resistance in Humans That Decrease Insulin Sensitivity In Vitro

Sphingolipids are thought to promote skeletal muscle insulin resistance. Deoxysphingolipids (dSLs) are atypical sphingolipids that are increased in the plasma of individuals with type 2 diabetes and cause β-cell dysfunction in vitro. However, their role in human skeletal muscle is unknown. We found that dSL species are significantly elevated in muscle of individuals with obesity and type 2 diabetes compared with athletes and lean individuals and are inversely related to insulin sensitivity. Furthermore, we observed a significant reduction in muscle dSL content in individuals with obesity who completed a combined weight loss and exercise intervention. Increased dSL content in primary human myotubes caused a decrease in insulin sensitivity associated with increased inflammation, decreased AMPK phosphorylation, and altered insulin signaling. Our findings reveal a central role for dSL in human muscle insulin resistance and suggest dSLs as therapeutic targets for the treatment and prevention of type 2 diabetes.

ARTICLE HIGHLIGHTS

Deoxysphingolipids (dSLs) are atypical sphingolipids elevated in the plasma of individuals with type 2 diabetes, and their role in muscle insulin resistance has not been investigated. We evaluated dSL in vivo in skeletal muscle from cross-sectional and longitudinal insulin-sensitizing intervention studies and in vitro in myotubes manipulated to synthesize higher dSLs. dSLs were increased in the muscle of people with insulin resistance, inversely correlated to insulin sensitivity, and significantly decreased after an insulin-sensitizing intervention; increased intracellular dSL concentrations cause myotubes to become more insulin resistant. Reduction of muscle dSL levels is a potential novel therapeutic target to prevent/treat skeletal muscle insulin resistance.

Sphingolipid desaturase DEGS1 is essential for mitochondria-associated membrane integrity

Sphingolipids function as membrane constituents and signaling molecules, with crucial roles in human diseases, from neurodevelopmental disorders to cancer, best exemplified in the inborn errors of sphingolipid metabolism in lysosomes. The dihydroceramide desaturase Δ4-dihydroceramide desaturase 1 (DEGS1) acts in the last step of a sector of the sphingolipid pathway, de novo ceramide biosynthesis. Defects in DEGS1 cause the recently described hypomyelinating leukodystrophy-18 (HLD18) (OMIM #618404). Here, we reveal that DEGS1 is a mitochondria-associated endoplasmic reticulum membrane-resident (MAM-resident) enzyme, refining previous reports locating DEGS1 at the endoplasmic reticulum only. Using patient fibroblasts, multiomics, and enzymatic assays, we show that DEGS1 deficiency disrupts the main core functions of the MAM: (a) mitochondrial dynamics, with a hyperfused mitochondrial network associated with decreased activation of dynamin-related protein 1; (b) cholesterol metabolism, with impaired sterol O-acyltransferase activity and decreased cholesteryl esters; (c) phospholipid metabolism, with increased phosphatidic acid and phosphatidylserine and decreased phosphatidylethanolamine; and (d) biogenesis of lipid droplets, with increased size and numbers. Moreover, we detected increased mitochondrial superoxide species production in fibroblasts and mitochondrial respiration impairment in patient muscle biopsy tissues. Our findings shed light on the pathophysiology of HLD18 and broaden our understanding of the role of sphingolipid metabolism in MAM function.

Diabetic vascular diseases: molecular mechanisms and therapeutic strategies

Vascular complications of diabetes pose a severe threat to human health. Prevention and treatment protocols based on a single vascular complication are no longer suitable for the long-term management of patients with diabetes. Diabetic panvascular disease (DPD) is a clinical syndrome in which vessels of various sizes, including macrovessels and microvessels in the cardiac, cerebral, renal, ophthalmic, and peripheral systems of patients with diabetes, develop atherosclerosis as a common pathology. Pathological manifestations of DPDs usually manifest macrovascular atherosclerosis, as well as microvascular endothelial function impairment, basement membrane thickening, and microthrombosis. Cardiac, cerebral, and peripheral microangiopathy coexist with microangiopathy, while renal and retinal are predominantly microangiopathic. The following associations exist between DPDs: numerous similar molecular mechanisms, and risk-predictive relationships between diseases. Aggressive glycemic control combined with early comprehensive vascular intervention is the key to prevention and treatment. In addition to the widely recommended metformin, glucagon-like peptide-1 agonist, and sodium-glucose cotransporter-2 inhibitors, for the latest molecular mechanisms, aldose reductase inhibitors, peroxisome proliferator-activated receptor-γ agonizts, glucokinases agonizts, mitochondrial energy modulators, etc. are under active development. DPDs are proposed for patients to obtain more systematic clinical care requires a comprehensive diabetes care center focusing on panvascular diseases. This would leverage the advantages of a cross-disciplinary approach to achieve better integration of the pathogenesis and therapeutic evidence. Such a strategy would confer more clinical benefits to patients and promote the comprehensive development of DPD as a discipline.

Long-term effects of l-serine supplementation upon a mouse model of diabetic neuropathy

Deoxysphingolipids (1-deoxySLs) are neurotoxic sphingolipids associated with obesity and diabetic neuropathy (DN) and have been linked to severity of functional peripheral neuropathies. While l-serine supplementation can reduce 1-deoxySL accumulation and improve insulin sensitivity and sensory nerve velocity, long-term outcomes have not yet been examined. To assess this, we treated 2 month old db/db mice, a model of DN, with 5-20 % oral l-serine for 6 months and longitudinally quantified the extent of functional neuropathy progression. We examined putative biomarkers of neuropathy in blood and tissue and quantified levels of small fiber neuropathy, looking for associations between lowered 1-deoxySL and phenotypes. Toxic 1-deoxySLs were suppressed long-term in plasma and various tissue including the sciatic nerve, which is particularly targeted in DN. Functional neuropathy and sensory modalities were significantly improved in the treatment group well into advanced stages of disease. However, structural assessments revealed prominent axonal degeneration, apoptosis and Schwann cell pathology, suggesting that neuropathy was ongoing. Hyperglycemia and dyslipidemia persisted during our study, and high levels of glutathione were seen in the spinal cord. Our results demonstrate that despite significant functional improvements, l-serine does not prevent chronic degenerative changes specifically at the structural level, pointing to other processes such as oxidative damage and hyperglycemia, that persist despite 1-deoxySL reduction.

Role of Impaired Glycolysis in Perturbations of Amino Acid Metabolism in Diabetes Mellitus

The most frequent alterations in plasma amino acid concentrations in type 1 and type 2 diabetes are decreased L-serine and increased branched-chain amino acid (BCAA; valine, leucine, and isoleucine) levels. The likely cause of L-serine deficiency is decreased synthesis of 3-phosphoglycerate, the main endogenous precursor of L-serine, due to impaired glycolysis. The BCAA levels increase due to decreased supply of pyruvate and oxaloacetate from glycolysis, enhanced supply of NADH + H⁺ from beta-oxidation, and subsequent decrease in the flux through the citric acid cycle in muscles. These alterations decrease the supply of α-ketoglutarate for BCAA transamination and the activity of branched-chain keto acid dehydrogenase, the rate-limiting enzyme in BCAA catabolism. L-serine deficiency contributes to decreased synthesis of phospholipids and increased synthesis of deoxysphinganines, which play a role in diabetic neuropathy, impaired homocysteine disposal, and glycine deficiency. Enhanced BCAA levels contribute to increased levels of aromatic amino acids (phenylalanine, tyrosine, and tryptophan), insulin resistance, and accumulation of various metabolites, whose influence on diabetes progression is not clear. It is concluded that amino acid concentrations should be monitored in patients with diabetes, and systematic investigation is needed to examine the effects of L-serine and glycine supplementation on diabetes progression when these amino acids are decreased.

The Dark Side of Sphingolipids: Searching for Potential Cardiovascular Biomarkers

Cardiovascular diseases (CVDs) are the leading cause of death and illness in Europe and worldwide, responsible for a staggering 47% of deaths in Europe. Over the past few years, there has been increasing evidence pointing to bioactive sphingolipids as drivers of CVDs. Among them, most studies place emphasis on the cardiovascular effect of ceramides and sphingosine-1-phosphate (S1P), reporting correlation between their aberrant expression and CVD risk factors. In experimental in vivo models, pharmacological inhibition of de novo ceramide synthesis averts the development of diabetes, atherosclerosis, hypertension and heart failure. In humans, levels of circulating sphingolipids have been suggested as prognostic indicators for a broad spectrum of diseases. This article provides a comprehensive review of sphingolipids' contribution to cardiovascular, cerebrovascular and metabolic diseases, focusing on the latest experimental and clinical findings. Cumulatively, these studies indicate that monitoring sphingolipid level alterations could allow for better assessment of cardiovascular disease progression and/or severity, and also suggest them as a potential target for future therapeutic intervention. Some approaches may include the down-regulation of specific sphingolipid species levels in the circulation, by inhibiting critical enzymes that catalyze ceramide metabolism, such as ceramidases, sphingomyelinases and sphingosine kinases. Therefore, manipulation of the sphingolipid pathway may be a promising strategy for the treatment of cardio- and cerebrovascular diseases.

Nontargeted and Targeted Metabolomic Profiling Reveals Novel Metabolite Biomarkers of Incident Diabetes in African Americans

Nontargeted metabolomics methods have increased potential to identify new disease biomarkers, but assessments of the additive information provided in large human cohorts by these less biased techniques are limited. To diversify our knowledge of diabetes-associated metabolites, we leveraged a method that measures 305 targeted or "known" and 2,342 nontargeted or "unknown" compounds in fasting plasma samples from 2,750 participants (315 incident cases) in the Jackson Heart Study (JHS)-a community cohort of self-identified African Americans-who are underrepresented in omics studies. We found 307 unique compounds (82 known) associated with diabetes after adjusting for age and sex at a false discovery rate of <0.05 and 124 compounds (35 known, including 11 not previously associated) after further adjustments for BMI and fasting plasma glucose. Of these, 144 and 68 associations, respectively, replicated in a multiethnic cohort. Among these is an apparently novel isomer of the 1-deoxyceramide Cer(m18:1/24:0) with functional geonomics and high-resolution mass spectrometry. Overall, known and unknown metabolites provided complementary information (median correlation ρ = 0.29), and their inclusion with clinical risk factors improved diabetes prediction modeling. Our findings highlight the importance of including nontargeted metabolomics methods to provide new insights into diabetes development in ethnically diverse cohorts.

d-Amino Acids and Classical Neurotransmitters in Healthy and Type 2 Diabetes-Affected Human Pancreatic Islets of Langerhans

The pancreatic islets of Langerhans are clusters of cells that function as endocrine units synthesizing and releasing insulin and a range of additional peptide hormones. The structural and chemical characteristics of islets change during type 2 diabetes development. Although a range of metabolites including neurotransmitters has been reported in rodent islets, the involvement of these cell-to-cell signaling molecules within human pancreatic islets in the pathophysiology of type 2 diabetes is not well known, despite studies suggesting that these molecules impact intra- and inter-islet signaling pathways. We characterize the enigmatic cell-to-cell signaling molecules, d-serine (d-Ser) and d-aspartate (d-Asp), along with multiple classical neurotransmitters and related molecules, in healthy versus type 2 diabetes-affected human islets using capillary electrophoresis separations. Significantly reduced d-Ser percentage and gamma-aminobutyric acid (GABA) levels were found in type 2 diabetes-affected islets compared to healthy islets. In addition, the negative correlations of many of the signaling molecules, such as d-Ser percentage (r = −0.35), d-Asp (r = −0.32), serotonin (r = −0.42), and GABA (r = −0.39) levels, with hemoglobin A1c (HbA1c) levels and thus with the progression of type 2 diabetes further demonstrate the disruption in intra- or inter-islet signaling pathways and suggest that these cell-to-cell signaling molecules may be potential therapeutic targets.

Type 2 diabetes disrupts circadian orchestration of lipid metabolism and membrane fluidity in human pancreatic islets

Recent evidence suggests that circadian clocks ensure temporal orchestration of lipid homeostasis and play a role in pathophysiology of metabolic diseases in humans, including type 2 diabetes (T2D). Nevertheless, circadian regulation of lipid metabolism in human pancreatic islets has not been explored. Employing lipidomic analyses, we conducted temporal profiling in human pancreatic islets derived from 10 nondiabetic (ND) and 6 T2D donors. Among 329 detected lipid species across 8 major lipid classes, 5% exhibited circadian rhythmicity in ND human islets synchronized in vitro. Two-time point-based lipidomic analyses in T2D human islets revealed global and temporal alterations in phospho- and sphingolipids. Key enzymes regulating turnover of sphingolipids were rhythmically expressed in ND islets and exhibited altered levels in ND islets bearing disrupted clocks and in T2D islets. Strikingly, cellular membrane fluidity, measured by a Nile Red derivative NR12S, was reduced in plasma membrane of T2D diabetic human islets, in ND donors’ islets with disrupted circadian clockwork, or treated with sphingolipid pathway modulators. Moreover, inhibiting the glycosphingolipid biosynthesis led to strong reduction of insulin secretion triggered by glucose or KCl, whereas inhibiting earlier steps of de novo ceramide synthesis resulted in milder inhibitory effect on insulin secretion by ND islets. Our data suggest that circadian clocks operative in human pancreatic islets are required for temporal orchestration of lipid homeostasis, and that perturbation of temporal regulation of the islet lipid metabolism upon T2D leads to altered insulin secretion and membrane fluidity. These phenotypes were recapitulated in ND islets bearing disrupted clocks.

Sphingolipids and their role in health and disease in the central nervous system

Sphingolipids (SLs) are lipids derived from sphingosine, and their metabolism involves a broad and complex network of reactions. Although SLs are widely distributed in the body, it is well known that they are present in high concentrations within the central nervous system (CNS). Under physiological conditions, their abundance and distribution in the CNS depend on brain development and cell type. Consequently, SLs metabolism impairment may have a significant impact on the normal CNS function, and has been associated with several disorders, including sphingolipidoses, Parkinson's, and Alzheimer's. This review summarizes the main SLs characteristics and current knowledge about synthesis, catabolism, regulatory pathways, and their role in physiological and pathological scenarios in the CNS.

Circadian rhythm of lipid metabolism

Lipids comprise a diverse group of metabolites that are indispensable as energy storage molecules, cellular membrane components and mediators of inter- and intra-cellular signaling processes. Lipid homeostasis plays a crucial role in maintaining metabolic health in mammals including human beings. A growing body of evidence suggests that the circadian clock system ensures temporal orchestration of lipid homeostasis, and that perturbation of such diurnal regulation leads to the development of metabolic disorders comprising obesity and type 2 diabetes. In view of the emerging role of circadian regulation in maintaining lipid homeostasis, in this review, we summarize the current knowledge on lipid metabolic pathways controlled by the mammalian circadian system. Furthermore, we review the emerging connection between the development of human metabolic diseases and changes in lipid metabolites that belong to major classes of lipids. Finally, we highlight the mechanisms underlying circadian organization of lipid metabolic rhythms upon the physiological situation, and the consequences of circadian clock dysfunction for dysregulation of lipid metabolism.

Serine Metabolism in Health and Disease and as a Conditionally Essential Amino Acid

L-serine plays an essential role in a broad range of cellular functions including protein synthesis, neurotransmission, and folate and methionine cycles and synthesis of sphingolipids, phospholipids, and sulphur containing amino acids. A hydroxyl side-chain of L-serine contributes to polarity of proteins, and serves as a primary site for binding a phosphate group to regulate protein function. D-serine, its D-isoform, has a unique role. Recent studies indicate increased requirements for L-serine and its potential therapeutic use in some diseases. L-serine deficiency is associated with impaired function of the nervous system, primarily due to abnormal metabolism of phospholipids and sphingolipids, particularly increased synthesis of deoxysphingolipids. Therapeutic benefits of L-serine have been reported in primary disorders of serine metabolism, diabetic neuropathy, hyperhomocysteinemia, and amyotrophic lateral sclerosis. Use of L-serine and its metabolic products, specifically D-serine and phosphatidylserine, has been investigated for the therapy of renal diseases, central nervous system injury, and in a wide range of neurological and psychiatric disorders. It is concluded that there are disorders in which humans cannot synthesize L-serine in sufficient quantities, that L-serine is effective in therapy of disorders associated with its deficiency, and that L-serine should be classified as a “conditionally essential” amino acid.

Effects of Isocaloric Fructose Restriction on Ceramide Levels in Children with Obesity and Cardiometabolic Risk: Relation to Hepatic De Novo Lipogenesis and Insulin Sensitivity

Sugar intake, particularly fructose, is implicated as a factor contributing to insulin resistance via hepatic de novo lipogenesis (DNL). A nine-day fructose reduction trial, controlling for other dietary factors and weight, in children with obesity and metabolic syndrome, decreased DNL and mitigated cardiometabolic risk (CMR) biomarkers. Ceramides are bioactive sphingolipids whose dysregulated metabolism contribute to lipotoxicity, insulin resistance, and CMR. We evaluated the effect of fructose reduction on ceramides and correlations between changes observed and changes in traditional CMR biomarkers in this cohort. Analyses were completed on data from 43 participants. Mean weight decreased (−0.9 ± 1.1 kg). The majority of total and subspecies ceramide levels also decreased significantly, including dihydroceramides, deoxyceramides and ceramide-1-phoshates. Change in each primary ceramide species correlated negatively with composite insulin sensitivity index (CISI). Change in deoxyceramides positively correlated with change in DNL. These results suggest that ceramides decrease in response to dietary fructose restriction, negatively correlate with insulin sensitivity, and may represent an intermediary link between hepatic DNL, insulin resistance, and CMR.

The Role of Sphingolipid Signaling in Oxidative Lung Injury and Pathogenesis of Bronchopulmonary Dysplasia

Premature infants are born with developing lungs burdened by surfactant deficiency and a dearth of antioxidant defense systems. Survival rate of such infants has significantly improved due to advances in care involving mechanical ventilation and oxygen supplementation. However, a significant subset of such survivors develops the chronic lung disease, Bronchopulmonary dysplasia (BPD), characterized by enlarged, simplified alveoli and deformed airways. Among a host of factors contributing to the pathogenesis is oxidative damage induced by exposure of the developing lungs to hyperoxia. Recent data indicate that hyperoxia induces aberrant sphingolipid signaling, leading to mitochondrial dysfunction and abnormal reactive oxygen species (ROS) formation (ROS). The role of sphingolipids such as ceramides and sphingosine 1-phosphate (S1P), in the development of BPD emerged in the last decade. Both ceramide and S1P are elevated in tracheal aspirates of premature infants of <32 weeks gestational age developing BPD. This was faithfully reflected in the murine models of hyperoxia and BPD, where there is an increased expression of sphingolipid metabolites both in lung tissue and bronchoalveolar lavage. Treatment of neonatal pups with a sphingosine kinase1 specific inhibitor, PF543, resulted in protection against BPD as neonates, accompanied by improved lung function and reduced airway remodeling as adults. This was accompanied by reduced mitochondrial ROS formation. S1P receptor1 induced by hyperoxia also aggravates BPD, revealing another potential druggable target in this pathway for BPD. In this review we aim to provide a detailed description on the role played by sphingolipid signaling in hyperoxia induced lung injury and BPD.

The etiology and pathogenesis of type 1 diabetes - A personal, non-systematic review of possible causes, and interventions

In this review after a lifelong research career, my personal opinion on the development of type 1 diabetes (T1D) from its very start to clinical manifestation will be described. T1D is a disease of an increased intestinal permeability and a reduced pancreas volume. I am convinced that virus might be the initiator and that this virus could persist on strategically significant locations. Furthermore, intake of gluten is important both in foetal life and at later ages. Disturbances in sphingolipid metabolism may also be of crucial importance. During certain stages of T1D, T cells take over resulting in the ultimate destruction of beta cells, which manifests T1D as an autoimmune disease. Several preventive and early treatment strategies are mentioned. All together this review has more new theories than usually, and it might also be more speculative than ordinarily. But without new ideas and theories advancement is difficult, even though everything might not hold true during the continuous discovery of the etiology and pathogenesis of T1D.

1-Deoxysphinganine initiates adaptive responses to serine and glycine starvation in cancer cells via proteolysis of sphingosine kinase

Cancer cells may depend on exogenous serine, depletion of which results in slower growth and activation of adaptive metabolic changes. We previously demonstrated that serine and glycine (SG) deprivation causes loss of sphingosine kinase 1 (SK1) in cancer cells, thereby increasing the levels of its lipid substrate, sphingosine (Sph), which mediates several adaptive biological responses. However, the signaling molecules regulating SK1 and Sph levels in response to SG deprivation have yet to be defined. Here, we identify 1-deoxysphinganine (dSA), a noncanonical sphingoid base generated in the absence of serine from the alternative condensation of alanine and palmitoyl CoA by serine palmitoyl transferase, as a proximal mediator of SG deprivation in SK1 loss and Sph level elevation upon SG deprivation in cancer cells. SG starvation increased dSA levels in vitro and in vivo and in turn induced SK1 degradation through a serine palmitoyl transferase-dependent mechanism, thereby increasing Sph levels. Addition of exogenous dSA caused a moderate increase in intracellular reactive oxygen species, which in turn decreased pyruvate kinase PKM2 activity while increasing phosphoglycerate dehydrogenase levels, and thereby promoted serine synthesis. We further showed that increased dSA induces the adaptive cellular and metabolic functions in the response of cells to decreased availability of serine likely by increasing Sph levels. Thus, we conclude that dSA functions as an initial sensor of serine loss, SK1 functions as its direct target, and Sph functions as a downstream effector of cellular and metabolic adaptations. These studies define a previously unrecognized "physiological" nontoxic function for dSA.

Deoxysphingolipids Upregulate MMP-1, Downregulate TIMP-1, and Induce Cytotoxicity in Human Schwann Cells

Sphingolipids are a heterogeneous class of lipids and essential components of the plasma membrane and plasma lipoproteins. Studies have shown that plasma deoxysphingolipid (DSL), a newly identified sphingolipid class, is increased in diabetic patients and associated with diabetic neuropathy. However, it remains unknown if there is a causal relationship between plasma DSL increase and diabetic neuropathy. Since matrix metalloproteinases (MMPs) play an important role in diabetic neuropathy by degrading extracellular matrix in the peripheral nervous system, we investigated the effect of DSLs on the expression of MMPs and tissue inhibitor of metalloproteinase (TIMPs), and cytotoxicity in human Schwann cells. We quantified protein secretion, gene expression, and collagenase activity, and performed cytotoxicity assays. Results showed that DSLs upregulated MMP-1, downregulated TIMP-1, and induced cytotoxicity in Schwann cells. Furthermore, we quantified DSLs in VLDL, LDL, HDL2, and HDL3 isolated from type 2 diabetes mellitus (T2DM) patients with or without neuropathy. Interestingly, lipidomic analysis showed that only HDL2 isolated from T2DM patients with neuropathy contains significantly higher level of DSLs than that isolated from T2DM patients without neuropathy. Additionally, results showed that HDL2 isolated from T2DM patients with neuropathy was more potent than that isolated from T2DM patients without neuropathy in upregulating MMP-1, downregulating TIMP-1, and stimulating collagenase activity in Schwann cell. Taken together, this study demonstrated for the first time a potential causal relationship between DSLs and diabetic neuropathy and that DSL-containing HDL2 from T2DM patients with neuropathy was more potent than that from T2DM patients without neuropathy in stimulating collagenase activity.

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.

Senescence and Type 2 Diabetic Cardiomyopathy: How Young Can You Die of Old Age?

Inflammation is well understood to be a physiological process of ageing however it also underlies many chronic diseases, including conditions without an obvious pathogenic inflammatory element. Recent findings have unequivocally identified type 2 diabetes (T2D) as a chronic inflammatory disease characterized by inflammation and immune senescence. Immunosenescence is a hallmark of the prolonged low-grade systemic inflammation, in particular associated with metabolic syndrome and can be a cause as well as a consequence of T2D. Diabetes is a risk factor for cardiovascular mortality and remodelling and with particular changes to myocardial structure, function, metabolism and energetics collectively resulting in diabetic cardiomyopathy. Both cardiomyocytes and immune cells undergo metabolic remodelling in T2D and as a result become trapped in a vicious cycle of lost metabolic flexibility, thus losing their key adaptive mechanisms to dynamic changes in O₂ and nutrient availability. Immunosenescence driven by metabolic stress may be both the cause and key contributing factor to cardiac dysfunction in diabetic cardiomyopathy by inducing metabolic perturbations that can lead to impaired energetics, a strong predictor of cardiac mortality. Here we review our current understanding of the cross-talk between inflammaging and cardiomyocytes in T2D cardiomyopathy. We discuss potential mechanisms of metabolic convergence between cell types which, we hypothesize, might tip the balance between resolution of the inflammation versus adverse cardiac metabolic remodelling in T2D cardiomyopathy. A better understanding of the multiple biological paradigms leading to T2D cardiomyopathy including the immunosenescence associated with inflammaging will provide a powerful target for successful therapeutic interventions.

Metabolism of HSAN1- and T2DM-associated 1-deoxy-sphingolipids inhibits the migration of fibroblasts

Hereditary sensory neuropathy type 1 (HSAN1) is a rare axonopathy, characterized by a progressive loss of sensation (pain, temperature, and vibration), neuropathic pain, and wound healing defects. HSAN1 is caused by several missense mutations in the serine palmitoyltransferase long-chain base subunit 1 and serine palmitoyltransferase long-chain base subunit 2 of the enzyme serine palmitoyltransferase-the key enzyme for the synthesis of sphingolipids. The mutations change the substrate specificity of serine palmitoyltransferase, which then forms an atypical class of 1-deoxy-sphinglipids (1-deoxySLs). Similarly, patients with type 2 diabetes mellitus also present with elevated 1-deoxySLs and a comparable clinical phenotype. The effect of 1-deoxySLs on neuronal cells was investigated in detail, but their impact on other cell types remains elusive. Here, we investigated the consequences of externally added 1-deoxySLs on the migration of fibroblasts in a scratch assay as a simplified cellular wound-healing model. We showed that 1-deoxy-sphinganine (1-deoxySA) inhibits the migration of NIH-3T3 fibroblasts in a dose- and time-dependent manner. This was not seen for a non-native, L-threo stereoisomer. Supplemented 1-deoxySA was metabolized to 1-deoxy-(dihydro)ceramide and downstream to 1-deoxy-sphingosine. Inhibiting downstream metabolism by blocking N-acylation rescued the migration phenotype. In contrast, adding 1-deoxy-sphingosine had a lesser effect on cell migration but caused the massive formation of intracellular vacuoles. Further experiments showed that the effect on cell migration was primarily mediated by 1-deoxy-dihydroceramides rather than by the free base or 1-deoxyceramides. Based on these findings, we suggest that limiting the N-acylation of 1-deoxySA could be a therapeutic approach to improve cell migration and wound healing in patients with HSAN1 and type 2 diabetes mellitus.

1-Deoxysphingolipids cause autophagosome and lysosome accumulation and trigger NLRP3 inflammasome activation

1-Deoxysphingolipids (deoxySLs) are atypical sphingolipids of clinical relevance as they are elevated in plasma of patients suffering from hereditary sensory and autonomic neuropathy (HSAN1) or type 2 diabetes. Their neurotoxicity is described best but they inflict damage to various cell types by an uncertain pathomechanism. Using mouse embryonic fibroblasts and an alkyne analog of 1-deoxysphinganine (doxSA), the metabolic precursor of all deoxySLs, we here study the impact of deoxySLs on macroautophagy/autophagy, the regulated degradation of dysfunctional or expendable cellular components. We find that deoxySLs induce autophagosome and lysosome accumulation indicative of an increase in autophagic flux. The autophagosomal machinery targets damaged mitochondria that have accumulated N-acylated doxSA metabolites, presumably deoxyceramide and deoxydihydroceramide, and show aberrant swelling and tubule formation. Autophagosomes and lysosomes also interact with cellular lipid aggregates and crystals that occur upon cellular uptake and N-acylation of monomeric doxSA. As crystals entering the lysophagosomal apparatus in phagocytes are known to trigger the NLRP3 inflammasome, we also treated macrophages with doxSA. We demonstrate the activation of the NLRP3 inflammasome by doxSLs, prompting the release of IL1B from primary macrophages. Taken together, our data establish an impact of doxSLs on autophagy and link doxSL pathophysiology to inflammation and the innate immune system.Abbreviations: alkyne-doxSA: (2S,3R)-2-aminooctadec-17yn-3-ol; alkyne-SA: (2S,3R)-2- aminooctadec-17yn-1,3-diol; aSA: alkyne-sphinganine; ASTM-BODIPY: azido-sulfo-tetramethyl-BODIPY; CerS: ceramide synthase; CMR: clonal macrophage reporter; deoxySLs: 1-deoxysphingolipids; dox(DH)Cer: 1-deoxydihydroceramide; doxCer: 1-deoxyceramide; doxSA: 1-deoxysphinganine; FB1: fumonisin B1; HSAN1: hereditary sensory and autonomic neuropathy type 1; LC3: MAP1LC3A and MAP1LC3B; LPS: lipopolysaccharide; MEF: mouse embryonal fibroblasts; MS: mass spectrometry; N3635P: azido-STAR635P; N3Cy3: azido-cyanine 3; N3picCy3: azido-picolylcyanine 3; NLRP3: NOD-like receptor pyrin domain containing protein 3; P4HB: prolyl 4-hydroxylase subunit beta; PINK1: PTEN induced putative kinase 1; PYCARD/ASC: PYD and CARD domain containing; SPTLC1: serine palmitoyltransferase long chain base subunit 1; SQSTM1: sequestosome 1; TLC: thin layer chromatography.

A reference map of sphingolipids in murine tissues

Sphingolipids (SPs) have both a structural role in the cell membranes and a signaling function that regulates many cellular processes. The enormous structural diversity and low abundance of many SPs pose a challenge for their identification and quantification. Recent advances in lipidomics, in particular liquid chromatography (LC) coupled with mass spectrometry (MS), provide methods to detect and quantify many low-abundant SP species reliably. Here we use LC-MS to compile a "murine sphingolipid atlas," containing the qualitative and quantitative distribution of 114 SPs in 21 tissues of a widely utilized wild-type laboratory mouse strain (C57BL/6). We report tissue-specific SP fingerprints, as well as sex-specific differences in the same tissue. This is a comprehensive, quantitative sphingolipidomic map of mammalian tissues collected in a systematic fashion. It will complement other tissue compendia for interrogation into the role of SP in mammalian health and disease.

Ether lipids, sphingolipids and toxic 1-deoxyceramides as hallmarks for lean and obese type 2 diabetic patients

AIM

The worldwide increase in obesity and type 2 diabetes (T2D) represents a major health challenge. Chronically altered lipids induced by obesity further promote the development of T2D, and the accumulation of toxic lipid metabolites in serum and peripheral organs may contribute to the diabetic phenotype.

METHODS

To better understand the complex metabolic pattern of lean and obese T2D and non-T2D individuals, we analysed the lipid profile of human serum, skeletal muscle and visceral adipose tissue of two cohorts by systematic mass spectrometry-based lipid analysis.

RESULTS

Lipid homeostasis was strongly altered in a disease- and tissue-specific manner, allowing us to define T2D signatures associated with obesity from those that were obesity independent. Lipid changes encompassed lyso-, diacyl- and ether-phospholipids. Moreover, strong changes in sphingolipids included cytotoxic 1-deoxyceramide accumulation in a disease-specific manner in serum and visceral adipose tissue. The high amounts of non-canonical 1-deoxyceramide present in human adipose tissue most likely come from cell-autonomous synthesis because 1-deoxyceramide production increased upon differentiation to adipocytes in mouse cell culture experiments.

CONCLUSION

Taken together, the observed lipidome changes in obesity and T2D will facilitate the identification of T2D patient subgroups and represent an important step towards personalized medicine in diabetes.

Increased 1-Deoxysphingolipids and Skin Barrier Dysfunction in the Skin of X-ray or Ultraviolet B Irradiation and Atopic Dermatitis Lesion Could Be Prevented by Moisturizer with Physiological Lipid Mixture

Background

Skin diseases characterized by epithelial barrier dysfunction show altered sphingolipid metabolism, which results in changes in the stratum corneum intercellular lipid components and structure. Under pathological conditions, 1-deoxysphingolipids form as atypical sphingolipids from de novo sphingolipid biosynthesis.

Objective

This study investigated the potential role of 1-deoxysphingolipids in skin barrier dysfunction secondary to X-ray and ultraviolet B (UVB) irradiation in vitro and in vivo. It was also evaluated changes in the expression of 1-deoxysphingolipids in lesional human skin of atopic dermatitis.

Methods

In this study, the changes in these 1-deoxysphingolipids levels of skin and serum samples were investigated in skin barrier dysfunction associated with X-ray and UVB irradiation in vitro and in vivo.

Results

Increased 1-deoxysphingolipids were observed in cultured normal human epidermal keratinocytes after X-ray irradiation. X-ray or UVB irradiation increased the production of 1-deoxysphingosine in a reconstituted 3-dimensional (3D) skin model. Interestingly, treatment with a physiological lipid mixture (multi-lamellar emulsion contained pseudoceramide), which can strengthen the epidermal permeability barrier function, resulted in decreased 1-deoxysphingosine formation in a reconstituted 3D skin model. Further investigation using a hairless mouse model showed similar preventive effects of physiological lipid mixture against 1-deoxysphingosine formation after X-ray irradiation. An increased level of 1-dexoysphingosine in the stratum corneum was also observed in lesional skin of atopic dermatitis.

Conclusion

1-deoxysphingosine might be a novel biomarker of skin barrier dysfunction and a physiological lipid mixture treatment could prevent 1-deoxysphingosine production and consequent skin barrier dysfunction.

The long chain base unsaturation has a stronger impact on 1-deoxy(methyl)-sphingolipids biophysical properties than the structure of its C1 functional group

1-deoxy-sphingolipids, also known as atypical sphingolipids, are directly implicated in the development and progression of hereditary sensory and autonomic neuropathy type 1 and diabetes type 2. The mechanisms underlying their patho-physiological actions are yet to be elucidated. Accumulating evidence suggests that the biological actions of canonical sphingolipids are triggered by changes promoted on membrane organization and biophysical properties. However, little is known regarding the biophysical implications of atypical sphingolipids. In this study, we performed a comprehensive characterization of the effects of the naturally occurring 1-deoxy-dihydroceramide, 1-deoxy-ceramide^Δ14Z and 1-deoxymethyl-ceramide^Δ3E in the properties of a fluid membrane. In addition, to better define which structural features determine sphingolipid ability to form ordered domains, the synthetic 1-O-methyl-ceramide^Δ4E and 1-deoxy-ceramide^Δ4E were also studied. Our results show that natural and synthetic 1-deoxy(methyl)-sphingolipids fail to laterally segregate into ordered domains as efficiently as the canonical C16-ceramide. The impaired ability of atypical sphingolipids to form ordered domains was more dependent on the presence, position, and configuration of the sphingoid base double bond than on the structure of its C1 functional group, due to packing constraints introduced by an unsaturated backbone. Nonetheless, absence of a hydrogen bond donor and acceptor group at the C1 position strongly reduced the capacity of atypical sphingolipids to form gel domains. Altogether, the results showed that 1-deoxy(methyl)-sphingolipids induce unique changes on the biophysical properties of the membranes, suggesting that these alterations might, in part, trigger the patho-biological actions of these lipids.

Altered plasma serine and 1-deoxydihydroceramide profiles are associated with diabetic neuropathy in type 2 diabetes and obesity

Recent studies suggest that the accumulation of atypical, 1-deoxysphingolipids that lack the C1 hydroxyl group may be associated with diabetic neuropathy (DN). We hypothesized that specific plasma 1-deoxysphingolipids associate with DN severity, and that alterations in plasma serine and alanine associate with 1-deoxysphingolipid elevation in patients with type 2 diabetes (T2D). We examined individual 1-deoxysphingolipid species using LC/MS/MS in plasma samples from 75 individuals including lean controls (LC, n = 19), those with obesity (n = 19), obesity with T2D without DN (ob/T2D, n = 18), and obesity with T2D with DN (Ob/T2D/DN, n = 19). We observed a step wise increase in 1-deoxydihydroceramides across these four groups (spearman correlation coefficient r = 0.41, p = 0.0002). Mean total concentrations of 1-deoxydihydroceramides, and most individual 1-deoxydihydroceramide species, were higher in ob/T2D/DN versus LC group (8.939 vs. 5.195 pmol/100 μL for total 1-deoxydihydroceramides p = 0.005). No significant differences in 1-deoxydihydroceramides were observed between the ob/T2D and ob/T2D/DN groups. l-alanine was higher and l-serine lower in ob/T2D/DN versus LC groups (326.2 vs. 248.0 μM, p = 0.0086 and 70.2 vs. 89.8 μM, p = 0.0110), consistent with a potential contribution of these changes to the observed 1-deoxysphingolipids profiles. 1-deoxydihydroceramides correlated inversely with leg intraepidermal nerve fiber density (CC -0.40, p = 0.003). These findings indicate that 1-deoxydihydroceramides may be important biomarkers and/or mediators of DN.

Association of 1-deoxy-sphingolipids with steatosis but not steatohepatitis nor fibrosis in non-alcoholic fatty liver disease

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is the most important cause of chronic liver disease in the western world. Steatosis can be accompanied by inflammation and cell damage (non-alcoholic steatohepatitis, NASH), and even liver fibrosis. Sphingolipids are a heterogeneous class of lipids and essential components of the plasma membrane and plasma lipoproteins. The atypical class of deoxy-sphingolipids has been implicated in the metabolic syndrome and type 2 diabetes.

AIM

To determine if circulating (deoxy)sphingolipids are associated with NAFLD and its different entities, steatosis, inflammatory changes (inflammation and ballooning) and fibrosis.

METHODS

Sphingolipids were analysed by LC-MS after hydrolysing the N-acyl and O-linked headgroups in plasma of obese adults who underwent a liver biopsy in suspicion of NAFLD.

RESULTS

Two-hundred and eighty-eight patients were included. There was no association between typical sphingolipids and NAFLD and its different entities. There was a significant association between the presence of steatosis and the concentrations of deoxy-sphinganine [exp(B) 11.163 with CI (3.432, 36.306) and p < 0.001] and deoxy-sphingosine [exp(B) 8.486 with CI (3.437, 20.949) and p < 0.001]. There was no association between these deoxy-sphingolipids and activity of the steatohepatitis, nor was there any association with fibrosis. Differences in deoxy-sphingolipids also correlated independently with the presence of the metabolic syndrome, but not diabetes.

CONCLUSION

Deoxy-sphingolipids are elevated in patients with steatosis compared to those without fatty liver, but not different between the different NAFLD subtypes, suggesting that deoxy-sphingolipid bases might be involved in steatogenesis, but not in the further progression of NAFLD to NASH nor in fibrogenesis.

Oxidative stress increases 1-deoxysphingolipid levels in chronic kidney disease

Chronic kidney disease (CKD) leads to deep changes in lipid metabolism and obvious dyslipidemia. The dysregulation of lipid metabolism in turn results in CKD progression and the complications of cardiovascular diseases. To obtain a profound insight into the associated dyslipidemia in CKD, we performed lipidomic analysis to measure lipid metabolites in the serum from a rat 5/6 nephrectomy (5/6 Nx) model of CKD as well as in the serum from CKD patients. HK-2 cells were also used to examine oxidative stress-induced sphingolipid changes. Totally 182 lipid species were identified in 5/6 Nx rats. We found glycerolipids, total free fatty acids, and sphingolipids levels were significantly upregulated in 5/6 Nx rats. The atypical sphingolipids, 1-deoxysphingolipids, were significantly altered in both CKD animals and human CKD patients. The levels of 1-deoxysphingolipids directly relevant to the level of oxidative stress in vivo and in vitro. These results demonstrate that 1-deoxysphingolipid levels are increased in CKD and this increase directly correlates with increased kidney oxidative stress.

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.

Role of ceramides in the pathogenesis of diabetes mellitus and its complications

Diabetes mellitus (DM) is a systemic metabolic disease that affects 463 million adults worldwide and is a leading cause of cardiovascular disease, blindness, nephropathy, peripheral neuropathy, and lower-limb amputation. Lipids have long been recognized as contributors to the pathogenesis and pathophysiology of DM and its complications, but recent discoveries have highlighted ceramides, a class of bioactive sphingolipids with cell signaling and second messenger capabilities, as particularly important contributors to insulin resistance and the underlying mechanisms of DM complications. Besides their association with insulin resistance and pathophysiology of type 2 diabetes, evidence is emerging that certain species of ceramides are mediators of cellular mechanisms involved in the initiation and progression of microvascular and macrovascular complications of DM. Advances in our understanding of these associations provide unique opportunities for exploring ceramide species as potential novel therapeutic targets and biomarkers. This review discusses the links between ceramides and the pathogenesis of DM and diabetic complications and identifies opportunities for novel discoveries and applications.

Sex differences in the association of sphingolipids with age in Dutch and South-Asian Surinamese living in Amsterdam, the Netherlands

BACKGROUND

Men have a higher risk for cardiovascular disease (CVD) early in life, while women have a higher risk later in life. The sex-related differences in CVD risk, especially by age, could be related to sphingolipid metabolism. We compared plasma sphingolipid concentrations and its increase by age in men and women.

METHODS

Plasma concentrations of 13 types of sphingolipids were measured by liquid chromatography-tandem mass spectrometry in a random subsample of 328 men and 372 women of Dutch and South-Asian Surinamese ethnic origin, participating in the HELIUS study. Sphingolipid concentrations were compared between men and women by age group (18-39, 40-55, and 56-70 years). Multiple linear regression was used to determine sex differences in age trends in sphingolipids stratified by ethnicity. Analyses were performed without adjustment and adjusted for body mass index (BMI) and waist circumference.

RESULTS

At age 18-39 years, sphingolipid concentrations were lower in women than those in men, but at age 56-70 years this was reversed. At higher age, women showed higher concentrations than men. In line, we observed a more rapid increase of sphingolipid concentrations by age in women than in men. The observed sex differences were not explained by BMI or waist circumference. Patterns of sex differences were similar across ethnic groups, although the strength of associations differed.

CONCLUSIONS

Mean sphingolipid concentrations increase more rapidly with age in women than in men. Therefore, plasma lipid concentrations of sphingolipids, although lower in women than in men at younger age, are higher in women than in men at older age.

1-Deoxysphingolipids, Early Predictors of Type 2 Diabetes, Compromise the Functionality of Skeletal Myoblasts

Metabolic dysfunction, dysregulated differentiation, and atrophy of skeletal muscle occur as part of a cluster of abnormalities associated with the development of Type 2 diabetes mellitus (T2DM). Recent interest has turned to the attention of the role of 1-deoxysphingolipids (1-DSL), atypical class of sphingolipids which are found significantly elevated in patients diagnosed with T2DM but also in the asymptomatic population who later develop T2DM. In vitro studies demonstrated that 1-DSL have cytotoxic properties and compromise the secretion of insulin from pancreatic beta cells. However, the role of 1-DSL on the functionality of skeletal muscle cells in the pathophysiology of T2DM still remains unclear. This study aimed to investigate whether 1-DSL are cytotoxic and disrupt the cellular processes of skeletal muscle precursors (myoblasts) and differentiated cells (myotubes) by performing a battery of in vitro assays including cell viability adenosine triphosphate assay, migration assay, myoblast fusion assay, glucose uptake assay, and immunocytochemistry. Our results demonstrated that 1-DSL significantly reduced the viability of myoblasts in a concentration and time-dependent manner, and induced apoptosis as well as cellular necrosis. Importantly, myoblasts were more sensitive to the cytotoxic effects induced by 1-DSL rather than by saturated fatty acids, such as palmitate, which are critical mediators of skeletal muscle dysfunction in T2DM. Additionally, 1-DSL significantly reduced the migration ability of myoblasts and the differentiation process of myoblasts into myotubes. 1-DSL also triggered autophagy in myoblasts and significantly reduced insulin-stimulated glucose uptake in myotubes. These findings demonstrate that 1-DSL directly compromise the functionality of skeletal muscle cells and suggest that increased levels of 1-DSL observed during the development of T2DM are likely to contribute to the pathophysiology of muscle dysfunction detected in this disease.

The noncanonical chronicles: Emerging roles of sphingolipid structural variants

Sphingolipids (SPs) are structurally diverse and represent one of the most quantitatively abundant classes of lipids in mammalian cells. In addition to their structural roles, many SP species are known to be bioactive mediators of essential cellular processes. Historically, studies have focused on SP species that contain the canonical 18‑carbon, mono-unsaturated sphingoid backbone. However, increasingly sensitive analytical technologies, driven by advances in mass spectrometry, have facilitated the identification of previously under-appreciated, molecularly distinct SP species. Many of these less abundant species contain noncanonical backbones. Interestingly, a growing number of studies have identified clinical associations between these noncanonical SPs and disease, suggesting that there is functional significance to the alteration of SP backbone structure. For example, associations have been found between SP chain length and cardiovascular disease, pain, diabetes, and dementia. This review will provide an overview of the processes that are known to regulate noncanonical SP accumulation, describe the clinical correlations reported for these molecules, and review the experimental evidence for the potential functional implications of their dysregulation. It is likely that further scrutiny of noncanonical SPs may provide new insight into pathophysiological processes, serve as useful biomarkers for disease, and lead to the design of novel therapeutic strategies.

Mini review: Lipids in Peripheral Nerve Disorders

Neurons are polarized cells whose fundamental functions are to receive, conduct and transmit signals. In bilateral animals, the nervous system is divided into the central (CNS) and peripheral (PNS) nervous system. The main function of the PNS is to connect the CNS to the limbs and organs, essentially serving as a relay between the brain and spinal cord and the rest of the body. Sensory axons can be up to 3 feet in length. Because of its long-reaching and complex structure, the peripheral nervous system (PNS) is exposed and vulnerable to many genetic, metabolic and environmental predispositions. Lipids and lipid intermediates are essential components of nerves. About 50 % of the brain dry weight consist of lipids, which makes it the second highest lipid rich tissue after adipose tissue. However, the role of lipids in neurological disorders in particular of the peripheral nerves is not well understood. This review aims to provide an overview about the role of lipids in the disorders of the PNS.

Diabetes and Metabolic Disorders and the Peripheral Nervous System

PURPOSE OF REVIEW

This article provides an up-to-date review of the manifestations of neuropathy seen in the setting of diabetes and other metabolic disorders.

RECENT FINDINGS

Although a number of metabolic disorders cause or are associated with peripheral neuropathy, the neuropathies associated with glucose dysregulation make up the vast majority of cases. Recent investigations have determined major differences in the neuropathies associated with type 1 and type 2 diabetes. Neuropathy in type 1 diabetes is closely linked to glycemic control, whereas neuropathy in type 2 diabetes is linked to dyslipidemia, central obesity, hypertension, insulin resistance, and glucose control. Although length-dependent axonal distal symmetric polyneuropathy is the most common clinical presentation, diabetes is also associated with acute, asymmetric, painless, and autonomic neuropathies.

SUMMARY

The prevalence of diabetes and metabolic syndrome is increasing across the globe. The need to recognize and treat the wide array of clinical manifestations of neuropathy detected in individuals with metabolic disorders will continue to grow. As a consequence, an increasing number of well-trained physicians who can manage these patients is needed. At present, treatment is largely focused on prevention and symptomatic management. Investments into funding for both basic and clinical science are necessary to bring novel therapeutic interventions into clinical practice.

Serine administration as a novel prophylactic approach to reduce the severity of acute pancreatitis during diabetes in mice

AIMS/HYPOTHESIS

Compared with the general population, individuals with diabetes have a higher risk of developing severe acute pancreatitis, a highly debilitating and potentially lethal inflammation of the exocrine pancreas. In this study, we investigated whether 1-deoxysphingolipids, atypical lipids that increase in the circulation following the development of diabetes, exacerbate the severity of pancreatitis in a diabetic setting.

METHODS

We analysed whether administration of an L-serine-enriched diet to mouse models of diabetes, an established method for decreasing the synthesis of 1-deoxysphingolipids in vivo, reduced the severity of acute pancreatitis. Furthermore, we elucidated the molecular mechanisms underlying the lipotoxicity exerted by 1-deoxysphingolipids towards rodent pancreatic acinar cells in vitro.

RESULTS

We demonstrated that L-serine supplementation reduced the damage of acinar tissue resulting from the induction of pancreatitis in diabetic mice (average histological damage score: 1.5 in L-serine-treated mice vs 2.7 in the control group). At the cellular level, we showed that L-serine decreased the production of reactive oxygen species, endoplasmic reticulum stress and cellular apoptosis in acinar tissue. Importantly, these parameters, together with DNA damage, were triggered in acinar cells upon treatment with 1-deoxysphingolipids in vitro, suggesting that these lipids are cytotoxic towards pancreatic acinar cells in a cell-autonomous manner. In search of the initiating events of the observed cytotoxicity, we discovered that 1-deoxysphingolipids induced early mitochondrial dysfunction in acinar cells, characterised by ultrastructural alterations, impaired oxygen consumption rate and reduced ATP synthesis.

CONCLUSIONS/INTERPRETATION

Our results suggest that 1-deoxysphingolipids directly damage the functionality of pancreatic acinar cells and highlight that an L-serine-enriched diet may be used as a promising prophylactic intervention to reduce the severity of pancreatitis in the context of diabetes.

Smith AA, Giles C, Huynh K, Cinel M, Mellett NA, Cadby G, Hung J, Hui J, Beilby J, Watts GF, Shaw JS, Moses EK, Magliano DJ, Meikle PJ. High-coverage plasma lipidomics reveals novel sex-specific lipidomic fingerprints of age and BMI: Evidence from two large population cohort studies

Obesity and related metabolic diseases show clear sex-related differences. The growing burden of these diseases calls for better understanding of the age- and sex-related metabolic consequences. High-throughput lipidomic analyses of population-based cohorts offer an opportunity to identify disease-risk-associated biomarkers and to improve our understanding of lipid metabolism and biology at a population level. Here, we comprehensively examined the relationship between lipid classes/subclasses and molecular species with age, sex, and body mass index (BMI). Furthermore, we evaluated sex specificity in the association of the plasma lipidome with age and BMI. Some 747 targeted lipid measures, representing 706 molecular lipid species across 36 classes/subclasses, were measured using a high-performance liquid chromatography coupled mass spectrometer on a total of 10,339 participants from the Australian Diabetes, Obesity and Lifestyle Study (AusDiab), with 563 lipid species being validated externally on 4,207 participants of the Busselton Health Study (BHS). Heat maps were constructed to visualise the relative differences in lipidomic profile between men and women. Multivariable linear regression analyses, including sex-interaction terms, were performed to assess the associations of lipid species with cardiometabolic phenotypes. Associations with age and sex were found for 472 (66.9%) and 583 (82.6%) lipid species, respectively. We further demonstrated that age-associated lipidomic fingerprints differed by sex. Specific classes of ether-phospholipids and lysophospholipids (calculated as the sum composition of the species within the class) were inversely associated with age in men only. In analyses with women alone, higher triacylglycerol and lower lysoalkylphosphatidylcholine species were observed among postmenopausal women compared with premenopausal women. We also identified sex-specific associations of lipid species with obesity. Lysophospholipids were negatively associated with BMI in both sexes (with a larger effect size in men), whilst acylcarnitine species showed opposing associations based on sex (positive association in women and negative association in men). Finally, by utilising specific lipid ratios as a proxy for enzymatic activity, we identified stearoyl CoA desaturase (SCD-1), fatty acid desaturase 3 (FADS3), and plasmanylethanolamine Δ1-desaturase activities, as well as the sphingolipid metabolic pathway, as constituent perturbations of cardiometabolic phenotypes. Our analyses elucidate the effect of age and sex on lipid metabolism by offering a comprehensive view of the lipidomic profiles associated with common cardiometabolic risk factors. These findings have implications for age- and sex-dependent lipid metabolism in health and disease and suggest the need for sex stratification during lipid biomarker discovery, establishing biological reference intervals for assessment of disease risk.

Use of preclinical models to identify markers of type 2 diabetes susceptibility and novel regulators of insulin secretion - A step towards precision medicine

BACKGROUND

Progression from pre-diabetes to type 2 diabetes (T2D) and from T2D to insulin requirement proceeds at very heterogenous rates among patient populations, and the risk of developing different types of secondary complications is also different between patients. The diagnosis of pre-diabetes and T2D solely based on blood glucose measurements cannot capture this heterogeneity, thereby preventing proposition of therapeutic strategies adapted to individual needs and pathogenetic mechanisms. There is, thus, a need to identify novel means to stratify patient populations based on a molecular knowledge of the diverse underlying causes of the disease. Such knowledge would form the basis for a precision medicine approach to preventing and treating T2D according to the need of identified patient subgroups as well as allowing better follow up of pharmacological treatment.

SCOPE OF REVIEW

Here, we review a systems biology approach that aims at identifying novel biomarkers for T2D susceptibility and identifying novel beta-cell and insulin target tissue genes that link the selected plasma biomarkers with insulin secretion and insulin action. This work was performed as part of two Innovative Medicine Initiative projects. The focus of the review will be on the use of preclinical models to find biomarker candidates for T2D prediction and novel regulators of beta-cell function. We will demonstrate that the study of mice with different genetic architecture and widely different adaptation to metabolic stress can be a powerful approach to identify biomarkers of T2D susceptibility in humans or for the identification of so far unrecognized genes controlling beta-cell function.

MAJOR CONCLUSIONS

The examples developed in this review will highlight the power of the systems biology approach, in particular as it allowed the discovery of dihydroceramide as a T2D biomarker candidate in mice and humans and the identification and characterization of novel regulators of beta-cell function.

Canonical and 1-Deoxy(methyl) Sphingoid Bases: Tackling the Effect of the Lipid Structure on Membrane Biophysical Properties

Compared to the canonical sphingoid backbone of sphingolipids (SLs), atypical long-chain bases (LCBs) lack C1-OH (1-deoxy-LCBs) or C1-CH₂OH (1-deoxymethyl-LCBs). In addition, when unsaturated, they present a cis-double bond instead of the canonical  Δ4-5 trans-double bond. These atypical LCBs are directly correlated with the development and progression of hereditary sensory and autonomic neuropathy type 1 and diabetes type II through yet unknown mechanisms. Changes in membrane properties have been linked to the biological actions of SLs. However, little is known about the influence of the LCB structure, particularly 1-deoxy(methyl)-LCB, on lipid-lipid interactions and their effect on membrane properties. To address this question, we used complementary fluorescence-based methodologies to study membrane model systems containing POPC and the different LCBs of interest. Our results show that 1-deoxymethyl-LCBs have the highest ability to reduce the fluidity of the membrane, while the intermolecular interactions of 1-deoxy-LCBs were found to be weaker, leading to the formation of less-ordered domains compared to their canonical counterparts-sphinganine and sphingosine. Furthermore, while the presence of a trans-double bond at the Δ4-5 position of the LCB increased the fluidity of the membrane compared to a saturated LCB, a cis-double bond completely disrupted the ability of the LCB to segregate into ordered domains. In conclusion, even small changes on the structure of the LCB, as seen in 1-deoxy(methyl)-LCBs, strongly affects lipid-lipid interactions and membrane fluidity. These results provide evidence that altered balance between species with different LCBs affect membrane properties and may contribute to the pathobiological role of these lipids.

Diabetic neuropathy: what does the future hold?

Frustratingly, disease-modifying treatments for diabetic neuropathy remain elusive. Glycaemic control has a robust effect on preventing neuropathy in individuals with type 1 but not in those with type 2 diabetes, which constitute the vast majority of patients. Encouragingly, recent evidence points to new metabolic risk factors and mechanisms, and thus also at novel disease-modifying strategies, which are desperately needed. Obesity has emerged as the second most important metabolic risk factor for neuropathy (diabetes being the first) from consensus findings of seven observational studies in populations across the world. Moreover, dyslipidaemia and altered sphingolipid metabolism are emergent novel mechanisms of nerve injury that may lead to new targeted therapies. Clinical history and examination remain critical components of an accurate diagnosis of neuropathy. However, skin biopsies and corneal confocal microscopy are promising newer tests that have been used as outcome measures in research studies but have not yet demonstrated clear clinical utility. Given the emergence of obesity as a neuropathy risk factor, exercise and weight loss are potential interventions to treat and/or prevent neuropathy, although evidence supporting exercise currently outweighs data supporting weight loss. Furthermore, a consensus has emerged advocating tricyclic antidepressants, serotonin-noradrenaline (norepinephrine) reuptake inhibitors and gabapentinoids for treating neuropathic pain. Out-of-pocket costs should be considered when prescribing these medications since their efficacy and tolerability are similar. Finally, the downsides of opioid treatment for chronic, non-cancer pain are becoming increasingly evident. Despite these data, current clinical practice frequently initiates and continues opioid prescriptions for patients with neuropathic pain before prescribing guideline-recommended treatments.